This is the standard Mk15 sensor bridge. It is fabricated like almost everything on the Mk15, from stainless steel. There are three separate green sensor brackets, held together at the top by the machined stainless lid support. The base that these brackets screw to is called the Centre Ring, and is made from high impact plastic. Around the outside of the Centre Ring is the O ring which seals the inner diameter of the scrubber cannister.
The natural rubber outer scrubber cannister seal can just be seen poking up over the centre section inner lip. 

This is the 15.5 sensor bridge. As you can see it's a one piece moulded plastic item. There is no separate centre ring, so it screws directly to the one piece moulded plastic centre section. You'll also notice that there is no Lid Support fitting on top.
The purple O ring in the background is the outer scrubber cannister seal which replaces the natural rubber seal used on the Mk15.

 When the 15.5 centre section is flipped over and the counterlung removed, you can see the horseshoe board. The six screws holding the sensor bridge are clearly visible.

The two rigs are laid side by side for comparison with the centre sections open and the scrubber cannisters removed. Of particular note is the annular port around the outside of the scrubber. This is where the gas flows down to reach the counterlung. On the standard Mk15 the port is an approximately 10mm gap all the way around, but on the 15.5 it is a series of slots approximately 6mm wide. Also of note on this picture is the different moulding of the electronics pod, and the high pressure lines to the guages. These can be seen as the two stainless braided lines running out the top of the Mk15, which go over the shoulders. These are flexible lines all the way from the regulators at the bottom of the rig. On the 15.5 there is hard piping from the regulators up to the top of the rig, then flexible lines run from there. Kevin has replaced his stock items with smaller more flexible Poseiden guages with readouts in Bar.

Scrubber cannisters are different, with the Mk15 holding 3.7Kg and the Mk15.5 holding around 4.2Kg of sorb. The Mk15.5 top has more holes for gas flow, but the bottoms of both units have the same hole layout. The Mk15.5 cannister is totally plastic whereas the Mk15 cannister has a plastic outer with stainless steel top and bottom.
The Mk15.5 cannister is filled by removing the top cover and pouring the sorb into the doughnut, then replacing the cover.

This is how the Mk15 seals are effected. The scrubber cannister is placed into the centre section as shown, then the natural rubber seal which is shown folded down, is brought up, closing off the gas flow between the centre section and the cannister.
The Mk15 cannister is filled through the small hole in the side which can be seen here with the chromed cap fitted.

Close up detail of the Mk15 cannister removed for filling, with the cap off and a load of Draegersorb inside.

And here is the Mk15.5 scrubber cannister sitting in its centre section. The purple O ring as seen in the sensor bridge photo above makes the seal between the cannister and the centre section. The purple O ring you can see here is the lid seal.
Also of note in this picture is the DSV hose connectors on the centre section. On the Mk15 they are machined stainless fittings which screw through the centre section and seal with an O ring, whereas on the Mk15.5 they are an integral part of the one piece moulded plastic centre section.

Next on the Mk15, the lid is fixed over the top, and the black rubber seal shown is stretched over, creating the outer seal.

And here is the Mk15.5 with its clear lexan lid in place. You can see the top of the scrubber cannister and the moisture absorbant pad on top. The purple safety ring can be seen holding down the stainless steel clip, two of which hold the centre section lid on.

Here are both units with their centre sections fully assembled and ready to dive. The Mk15 metal lid is fixed by screwing down the bolt in the centre, whereas the Mk15.5 clear lexan lid is held down by stainless clips, and secured by the purple O ring.
OK, my Mk15 isn't quite 'standard' as it has fixed regs and is hard plumbed.

These are the two types of secondary display used on this type of rebreather. At the top is the unpowered analogue Mk15 secondary, which is also fitted to the Mk15.5 and variants. The lower unit is the digital Mk16 secondary. This secondary can be used with the Mk15/15.5 etc, and will show PPO2 readings but not primary battery voltage. This unit requires four 'N' size cells for power. It scrolls through the sensors one by one. This picture shows the analogue on sensor three, and the digital on sensor one, both reading air. The analogue unit is on all the time, and the digital unit is turned on by screwing down the knurled stainless steel ring/cover on the left until the contact and seal is made.

Here we have the standard US Navy Mk15 electronics. At the bottom right is the selector switch for selecting between sensors one, two, three, or an average of all three. The three potentiometers are for adjusting the calibration point of each sensor.

This is the Juergensen/Smithers electronics as fitted to most Mk15.5's. In the background is the green circuit board, the lower left screw head is the five position adjustment switch to change setpoint. Upper right is a push button to confirm the loop PPO2 reading when calibrating, and the five white stripes are put on to correspond to where the screw head would align to for the five different setpoints. Setpoints available are 0.4, 0.7, 1.0, 1.2, and 1.4. The upper stripe coresponds to 1.4, the lower stripe to 0.4.

The Mk15 and Mk15.5 are fitted with different internal weights. Here the two weights are laid out to show the difference. The weight on the left is from the Mk15, and the one on the right is from the Mk15.5.

This picture shows the Mk15 weight mounted in its position in the chassis. This weight weighs around 3.5lbs.

And this picture shows the Mk15.5 weight in its position. This weight weighs around 4lbs.

After experimenting with external weights to take the bum-heavyness out of my unit, I've mounted both weights internally. This setup gives a perfect balance.