Hi, Dave,
Those VM Workshop tapes that have "PLC" in there were created with VMFPLC2 ... VMFPLC2 was used by IBM to create distribution tapes, and uses a better blocksize than the ordinary CMS TAPE command.??
So, for those Workshop tapes, just use VMFPLC2 instead of TAPE (or PETAPE in the case of the older WATERLOO tapes).
Hope that helps,
Mark S. Waterbury