I posted these part numbers for the "B28" vacuum advance unit before, but I can't locate the post; ALL vacuum advance units are manufactured exclusively by Standard Motor Products, who sells them to branding wholesalers who re-box them with their own part numbers (but ALL are stamped by SMP with "B"-codes, which define their characteristics).
"B28" Vacuum Advance Units (start at 4" Hg., finish at 8" Hg.):
GM #88924985
AC-Delco #D1312C
Airtex #4V1053
BWD #V329
Standard #VC-177
Wells #DV1810
Niehoff #DR-305
Car Quest #57-7537
NAPA/Echlin #VC-1810