It is my impression that Scubapro may change the parts in a service kit fairly often, perhaps annually. I suspect this is due to failure patterns revealed by parts orders and known wear patterns of service type parts. These changes generally do not involve a service kit p/n change. the change from 041 to 045 may just have been to put a clear line in the sand. Often it involves a change in packing container but sometimes it is just different parts that show up.
The current Mk25 kit seems to allow replacement of all o-ring. Previous kits had a much lower o-ring count as most static o-rings were expected to be reused. In the late '90s, the 10-750-041 was for the Mk20 and included the 1 LP piston o-ring and the plastic back up ring used at the time plus a few other parts. As Scubapro moved to the Mk25 and the composite piston upgrade was applied to Mk20s that were having problems with the older pistons, the BU rings was replaced with the 2 LP piston o-rings now found in almost all Mk20/25 but the p/n remained -041.
If you are having to scrounge for parts kits, you may need to take a peak inside to see if all the things you need are included. Heck, there was a time when the G250 kit included both the S-wing poppet assembly and a new balance chamber. That was great for those of us upgrading old Balanced Adjustables. Older G200 (109) kits used to include the duro poppet assembly - great for DIYers upgrading 109s. Then, for a spell they just included the new LP seat. Now the -045 G200 kit includes the duro poppet assembly again. DIYers looking to upgrade 109 need to seek out those kits so separate sourcings of duro poppets can be avoided.
