I have a Sig PM400 with the Sig brace and I love it. Easy to shoot from the shoulder and with the brace attached to your forearm. If you make it to Tulsa you are welcome to shoot it.
They were just a sequence number. Mine is stamped TPD 955. When TPD swithched over to the 4046 in the early 90's, I bought mine back for the sum of $ 139.00.