1980 Flag Stamps Will Not Soak Off

I received a large batch of precancels many years ago, arranged in piles with the perfs 100% aligned and in stacks of about 8-10 stamps.

When I opened them, I found every stack was completely stuck together (rough guess, 20 stacks, plus some blocks of four equally stuck to each other). I was busy at the time, so I put them aside for "one day". Yesterday was that day. I spent the best part of 12 hours soaking and separating those stamps. Many were extraordinarily difficult to separate. I thinned at least 4 when I thought I had them moving apart and they actually weren't.

By midnight, I was down to 4 obdurate pairs that absolutely would not separate. The top stamp (therefore the one whose gum was holding the pair together) in each case is one of two, issued in 1980:

18c flag with "fields of amber grain" Scott1890, pfs on all 4 sides
20c flag with Supreme Court, Scott 1894(?), pfs on all 4 sides

Both of these obviously pre-date the modern "sticky gum, won't soak" period.

I dried them overnight and today I tried soaking them in Stamp Lifting Fluid.

I got one pair apart, eventually, but thinned the flag stamp (the glue still held tighter than the paper could stand).

The other three pairs are still soaking in the Lifting fluid (over an hour now).

Questions:

a) any clue why this happened with these stamps? Was USPS experimenting with some new-fangled gum on these issues?

b) any suggestions for something better to try to get them apart?

With the perfs so perfectly aligned, there are two pairs I can't even poke at with a fingernail to get a hold.

Many thanks.

I use fingernail polish remover to remove the stamps from envelope. Try applying the polish remover to the backside of stamp then try removing it. I do not know how it will work on stacks of stamps. It will work on removing modern self sticking stamps from envelopes. I apply to the reverse side stamp face down. Letting the polish remove soak threw to he back of the stamp. What it may do to the face of the stamp I do not know. If you are going to try this I would do so in a limited amount to see how it is going to work. I hope this works for you. Please keep us informed to what methods works best.

After 23 hours in the Stamp Lift Fluid, the three remaining pairs of stamps came apart. Two pairs separated perfectly, one caused a miniscule thin.

I will try your nail polish remover tactic, one stamp at a time, thank you!

Acetone (nail polish remover) can affect some inks so be warned.