I'm always a little hesitant when it comes to the authenticity of movie props, unless you have documentation that these props were in fact used in the movie, there is no real way to verify them. While Hans (Alan Rickman) does use a P7 M13 in Die Hard it is nickel plated, not brown. Brigitte Nielson's in BHC II is black as shown in these photos
Also you mentioned the date on the handle as being '87. While BHC II was released in '87, principal photography began in '86. Die Hard on the other hand released in the summer of '88 but was shot in '87. Most likely the prop master would have marked the gun's date when he made it ('87 for Die Hard, '86 for BHC) not when the movie was to be released, since there is no guarantee when or even if a movie will be released. Also, just because there wasn't a brown M13 in Die Hard doesn't mean they didn't use one on set. As a filmmaker I can tell you that kind of thing happens quite often. Many times prop masters will have several versions of a gun available and the Director will choose which "look" he likes best for the film. It also could have been the gun of another character in the film and its use was simply cut out of the film. It could have even come from another film entirely. Props bounce around studio lots and are used on multiple films all the time. For instance Willis' Beretta 92FS used in Die Hard 2 and 3 makes a "cameo" appearance in Live Free or Die Hard, used by one of the bad guys (Willis switched to a Sig in that movie). But like I said, unless you have a certificate of authenticity or something like that there's no way to know for sure where your gun came from.
As for the markings, the prop master likely copied them right off of a real P7 M13 (probably one that was actually fired in the film it was used in, so every detail on the prop gun could match). So all the markings seem legit but don't have any special significance. The "IE" is the date code on the gun with A=0, B=1, etc... so in this case the gun they used to copy the markings would have been made in '84.
Hope that helps and sorry about the long rant :)