You should very carefully check the OD of these slugs one by one!
They may be sequentially packaged in increments.
Could be for testing lands diameters in barrel manufacturing?

When you said "Frankford Arsenal" I thought you meant the current budget priced company making inexpensive tools.
Coming from the real place adds significant interest.

You could inquire at the CMP forum, where there is considerable interest in period correct 1911 minutae.

Jim