It could be either as that was the configuration of the Mk 20 when they switched to the adjustable seat retainer to allow external IP adjustment.
The Mk 20 evolved over time with various detail changes such as small round, large round and finally oval ambient pressure holes, different trim pieces, two different styles of LP port turret, three different types of pistons, changes to the TIS system, etc. One of the detail changes was the new adjustable seat retainer and when that occurred Scubapro decided it was time to call it a Mk 25. So other than the seat retainer, the last Mk 20 and the first Mk 25 are exactly the same.
The Mk 25 has in turn went through several detail changes including larger oval ambient pressure holes, a minor change in the piston stem, more changes to the TIS system, a restyled finned swivel cap and new LP port turret stlying - all of the latter designed to improve heat tranfer and make it more relaible in cold water.
Pop the trim boot off the end near the yoke. If the seat retainer is round with a recess for a large allen wrench it is a Mk 20. If the seat retainer is hex shaped with a small hole in the middle for a small allen wrench, it is a Mk 25 - or it is a very late production Mk 20 that has been converted to a Mk 25. Either one may have shims as the IP was intended more for precision adjustment and shims are still used inside and out.
