This answer is not what you want to hear. When you take a picture, a latent image is formed within the silver. That latent image is very volatile. It is termorary at best. That is why they always tell you to process quickly. Thee oldest recovered films I've ever heard of were from the teens,--film from an arctic expedition were found up there some 70-80 years after the images were formed. The films were processed and some images were still there. But, the difference is substantial. Those films were lost in a freezing climate and basically sat in the equivalent of a freezer for all those years. That does make a difference.(posted 8711 days ago)With the C-41 process, and a latent image from the 60s or even 70s,-- you will essentially find nothing. Of course, processing is cheap so just have them go for it,--using some of the techniques, above. However, don't bother to shoot up the rest of the roll. That will simply confuse the processers when they clip an end to test the proper recovery for it.
dan