I'll have to get back to you with some actual numbers but I can tell you that the 19.7gr I measured before was fired, unsized mixed headstamp. I have checked any of the current batches I'm shooting.
As a point of information, I don't recall ever seeing a max load with 124/5gr bullets higher than 13.0gr (or lower, I think. It's actually remarkably consistent, considering some variance in other data) and EVERY source I've seen lists it as compressed. It's also universally (or nearly so) accepted/expected to be a compressed charge, as it is the "standard" answer to solving bullet setback issues in the Sig.
So, whatever case capacity results in 13.0gr being compressed is probably very close to correct.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.