Well, not cure about Shakespeare brand bows but for other makers, usually a 45X = 46 pounds, 45XX = 47pounds, x45 = 44 pounds, xx45 = 43 pounds etc. Meaning, they were going for a 45lb bow and due to uncontrollable things like individual pieces of wood bending different, ended up just over or under the target weight and it was marked accordingly.
You say it is marked +45+ and I don't know what that is unless it means it is within a pound + or - of 45?
Also, I don't think it would be 45# @ 29", rather if it is marked AMO standard, it would be 45# @ 28" and the model is the X29 but I am guessing a bit.
I did a quick google search and found this site:
http://www.archeryarchives.com/Sshakespeare.htm Looks like there was definitely a model X-29 bow. If you pull up different years, you may be able to determine about when your bow was made.