OK, so Brian from Able Ammo never emailed me like I suspected. I called Houston and spoke to Patrick who is in charge of these numbers.

In CIP standards:

TLT28SUB = 6900 psi
This is 1 ounce, 12 gauge subsonic at 1050 fps.

TLSK28 & TLT28 = 7900 psi
These are 12g., one ounce, skeet and trap loads that are the same except for shot size.

If you add 600 to 1,000 psi to reach American psi, they are rather marginal for all but the toughest vintage guns.

So many guns, so little time!