Soaking Hungarian Stamps From Kiloware

Hi all,
some time ago I got my first real kiloware and must say I am quite satisfied. Many countries (world wide ), some parts including FDC's,
some stamps still on paper, some already washed.
Now I begin with Hungary and find that there are many stamps from the 1920s which just refuse to separate from the paper they are sticking to. Had them in warm water for 1 1/2 hours and nothing budging.
Now they are in a drying book with the paper.
Has anybody an idea which adhesive was used for stamps in those times of need? Depending on that, how to get the stamps from the paper?
Thank you and hoping for good news
Heinrich

I haven't had that problem, but I have rarely soaked Hungarian stamps from that time. Anything I have on paper is on cover/postcard and I'm not soaking them to find out. Are they any one particular issue or type of paper?

Hello HungaryForStamps and all other interested,
here is a scan of those resisting fellows.



Thank you!