On the old A & B size cases, I forget for positive which was which but one was sized to fit normal chambers & required the oversized wads. Many of the older guns built for their use will have oversize bores as well. The other size required the gun be ordered special chambered with an undersize chamber. The thin wall hull was sized for use of regular wads in a standard bore. Not sure but I think the A fit the regular chamber & the B was for the special size. I can't recall that I have ever seen one of the special cases. I suspect, but do not know, that guns other than Parker Bros could be ordered with this chambering.
