I also have lots of OSSMB but no OSSMA.
The OSSMB is the only version I've ever seen, and AFAIK it tells you all
there is to know (or rather, all that is known) about calling OS/8 "system
services", including USR.
Also, OSSMB says that it is for OS/8 version 3, so that may imply that the
OSSMA was just an older version.
There are several copies of OSSMB online - check
vt100.net/manx for a
list.
Bob
P.S. What are you going to write? I've got some suggestions if you need
ideas :-)