Re: 1940s studio photo glued to mat
It would be my guess that it is not actually glue that binds it to the matting but I am not sure if they had created dry mounting tissue at that time. I still find it hard to believe that a photographer would use simple glue so here are my suggestions on removing the print. I have done quite a few and also helped some on here.
If it is indeed dry mounting tissue it is a piece of paper much like wax paper that sandwiches between your print and the mat board then placed in a heat press that slightly melts the tissue adhering it to the base and photo. A good number of times if you slightly twist the photo holding opposite corners (one on bottom right one on top left), time and poor mounting will have caused some of the glue to lose it's stick and the print may pop off on a corner. This is a good sign that it is indeed mounting tissue. If so, place the print on a cookie sheet, as low as your oven will go and set it in there with the door open. Check it often and eventually you should see the print begin to lift. You can also use a hair dryer if you blow the hot air on the back of the board and not the photo.
If all that fails and the following is true - #1) The photo paper is still in good shape, maybe yellowed but not torn badly or anything. It is not a black and white photo that has been hand colored with paints like the good old days. Try the following. Fill a clean, non-rusty baking pan or casserole dish with luke warm water. Submerge the entire image, board and all. After a while you should see the print start to lift. You can help it GINGERLY but don't force it. The print may turn funny colors and such but if you lay it on a drying rack or similar it will return to normal colors. (If it is hand colored the dyes will lift into the water). I hope this helps and I am around if you have more questions.