The guess that the magazines will be 15 round unless specifically stated otherwise may be correct. But the facts available to me do not lead me to such a conclusion.
I bought a SR9 and was bitterly disappointed to find that it was with 10 round mags-- kicked myself for not specifically asking.
My local dealer has found 44 of the Gen 4 Model 20 with 10 round mags but none yet with 15. The 10 round is plentiful, the 15 round is scarce -- based on the experience here.
The local price is a bit better than the offering on Gunbroker so I have not bothered to inquire about those specific listings. But I would do so before bidding.