It’s been a long time and my memory isn’t as sharp as it once was, but hopefully Billy Edwards will show up before long and either confirm or dispute what follows. I defer to him in advance.
As I think someone said, P is a proprietary multiplex sectional series (PA thru PH). While there is a PMK blank that passes all P multiplex keyways, there is no corresponding keyway that accepts only the PMK blank. The is also an L proprietary sectional series and LMK blank.
Page 21 in the document Pete referenced in post #15, shows the G, S and T multiplex keyways, their associated xMK blanks and a GST blank that passes ALL the G, S and T multiplex keyways. Note that while each group has a xMK blank, the GST blank is not marked GSTMK.
Not shown are GS, ST and GT blanks, each passing the indicated groups of keyways. They exist, but are rarely used. Again, take notice of the 2-letter indicators without the trailing MK.
Take a close look at the pictures of the RN11PL blanks from post #4. It could be these old eyes, but they appear to be multi-section MK blanks; probably passing all P and L multiplex keyways.
