The original Accurate Arms 3100 was simply relabeled surplus IMR-4831. No lot numbers were shown - they didn't need to as there was only a single huge lot.
When it was gone, and I can't give a date, Accurate Arms sourced a replacement from Explosia A.S. in what was then Czechoslovakia, now the Czech Republic. This powder is still available in Europe as Lovex SO71. It is (very) roughly equivalent to IMR-4831 and is a rather coarse-cut slow burning tubular powder.
When Western bought AA out 16 years ago, it terminated the Czech contract and has failed to source a lasting replacement for 3100 from any of its existing suppliers. Shooters World is now buying Lovex powders in bulk, renaming them and packaging them up in the US for distribution. So far, it hasn't adopted SO71 as one of its standard grades, although it's been reported on this forum that they've imported and sold bulk lots to ammunition companies. People have asked SW to consider adding SO71/3100 to their canister range, but have no idea if this will ever happen.
What do you load for? I have the old (2000) Accurate reloading manual pre-dating the Western changes and may be able to help.