Well, there's
http://audioheritage.org/intro.htm
to look at, though I don't see much about transformers.I have an old Peerless catalog. S-256-Q is 40 watts rating,
primary 5000 ohms CT, secondary taps for 16, 12, 8, 4 and
2 ohms. Max DC in primary is 120 mA balanced, 12 mA unbal.
S-258 isn't in here, sorry. I THINK it is the same except
with UL taps at 25%, or 1250 ohms.