OK - I just checked my stock of filters. I have five filters. All of them are Purolators. Two boxes have the number P-51 on the top of the boxes and three have the number P51-L20051. ALL of the filters have the P-51 number on the side of the filter itself. It also has a round ring which you use to pull the used filter out of the Beehive housing attached to the top of the filter. ALL filters are 4-3/4" tall and 4" wide. I bought ALL of these filters at Pep Boys because that is the only place I have ever seen them. I over buy because that's what I do especially if something like what has happened to you could happen to me.
So, It sounds to me like either something has changed since I have bought filters or the boxes you are buying are mis-marked.
What I can do in order to try to help you out and find out what is going on is go to my local Pep Boys and see what they have to offer in the way of a P51-L20051 since there appears to be something going on with this filter number. I'll check on it tomorrow and let you know what I find.
