The old bag molds show up on ebay regularly. They have a number code on them. The full list goes from 1 to 150. Some common numbers tied to common calibers are
150-.314"-32 cal.,
108-.350"-36 cal.,
75-.396-40 cal.,
58-.431-44 cal.,
59-.429-44 cal.,
53-.444-45 cal.,
39-.492-50 cal.
31-.531-54 cal.,
25-.571-58 cal.,
22-.596-60 cal.,
20-.615-62 cal./20 ga.
15-.677-69 cal.
12-.729-75 cal.
8-.835-8 bore
4-1.052-4 bore
Those were what I have on the list as some of the most common numbers on the molds. Hope that helps. If anyone has one marked 35 which is .510" I'd be interested, as I have a 52 cal. Al Martin in the works.
Bill