Two things to try. The safest thing would be to take these to a photo studio. Not the 1 hour things, but a real photo shop, the kind that supplies artists. Usually that guy can provide info on the papers and glues and what can or can't be done safely. The second way, takes the old fashioned method of steaming. You know, hold the "envelope" over the steam from a kettle and it gums up the glue and it can be opened without ripping the paper. Well, it does work, but can't say what it does to old dry glue, not to mention that it takes hot water and throws it on old dry paper and may cause damage to the writing. A final method, is to hold the page up to a bright light, allowing the light to shine through and see if it reveals the message.