They most likely are nitrocellulose based film. Acetate films did not come until much later. If they are not brittle dont worry about it, they should be fine.
Of course a way to test is to snip a small part of the negative and IN A SEPARATE room light it up, if it burns fast, you know for sure.
If the stain is uniformly yellow, it is most likely the processing done, they used stanning developers almost exclusively at that time.
Have fun and post some of the images.