odd sized rear window

[cknfrmr]

I need some info on rear window PLEASE. So I took out the rear window on my 69 c10 short bed fleetside to replace window seal. Went to put new seal in and its was way bigger than the window so I thought by chance at factory wrong seal got put in package. Went to classic Chevy parts store and they pulled new one out of package and checked numbers and all matched up. So I had old one with me and showed it to them and we measured it and everything. Come to find out there is a odd sized window in it. 56 x 14 glass. Does anyone know anything about this? They made about 5 calls and said non of manufactures heard of that. Don't see that has been chopped. That was one of their first questions. Any light you can shine on this would be great. Checked VIN number identification on here and couldn't see anything out of ordinary. Just don't know. Thanks for your time.