LCromwell,
The internals of the SE3 2nd stage are mostly the same as an SE2 second stage, except for the valve body which is metal and is removable from the plastic housing (not molded in to the plastic housing as on the SE2). It takes the same service kit as an SE2, and uses all the o-rings in the kit including the 2-015 o-ring for the removable valve body (now you guys with SE2s know what the extra o-ring in the kit is for).
The service port plug holds down an alignment key that keeps the diaphragm cover from being unscrewed. It's easier to open up (without the key) than an SE2, because the cover sticks out farther and comes out flush with the sides of the housing, and is much easier to grip. The plug takes a larger 8mm allen wrench than the 5/64" wrench for an SE2, so it won't round-out as easliy. Happily, it's the same wrench that fits the thermo DIN/K valve inserts, so that's 1 less allen wrench in the save-a-dive kit. The same Peterbilt special tools that fit all the conshelfs (poppet holder-downer, and 1/4" hollow nutdriver with screwdriver inside) fit this reg.
The purge button is a little larger than on an SE2, making it easier to push (or get pushed). It breathes the same as an SE2 (as it should -- it has the same guts). The SE3 1st stage is identical to an SE2.
Hope this helps,
Ltstanfo