I've got a bunch of old sockets like that. I think I finally found the answer in some of my old catalogs.
Seems to be that there was a handful of "standards". The old 19, 25, and 31 correspond with the old U.S. standard. The 23 and 29 correspond with the old Mfr. standard. The 15, 21, and 27 correspond with the old Mfr. standard for square bolt heads and nuts.
Maybe this will help......
19/32 = a 5/16 hex bolt by US standard
25/32= a 7/16 hex bolt ""
31/32= a 9/16 hex bolt ""
17/32= a 5/16 hex bolt by Mfr. standard
23/32= a 7/16 ""
29/32= a 9/16""
15/32= a 5/16 SQUARE bolt by Mfr. standard
21/32= a 7/16 ""
27/32= a 9/16 ""
These are out of a mid 50's New Britain catalog but other old catalogs have same info.
An early 70's Snap on catalog no longer has the sizes in the chart. They have a column for Old U.S. and old SAE but no 32nd sizes.
They have a date for the American Standard Association sizes and that shows 1965.
To sum up.....they consolidated bolt sizes in the 60's and that is when the odd sizes disappeared. Unless you are working on something old you probably won't need one....at least not for it's intended application.
rjohnson......Are the planes older than mid 60's? I suppose aircraft could have lingered on but I would think the standard would be across the board since the Snap on catalog didn't list them......but they continued to sell them for a long time.