After researching the issue there seems to be a final decision by the BATF that you do indeed have to mark all SBR/SBSs created using a BATF Form 1.
In this thread both the original letter from the BATF saying no markings required and a follow up letter that says it is required is shown:
http://www.uzitalk.com/forums/showthread.php?t=17660
Sorry about the bad info.
Rob