Early gas port barrels were dated S-A-6-40 and used revision 2 receivers around the 50K range - maybe some in the very late 40K range.
photo shows 52112 with S-A-7-40 bbl, original springs and SA SPG stock (some restoration was done)
The last gas traps were over the 50K serial range and used revision 2 receiversAttachment 68486Attachment 68487Attachment 68488Attachment 68489