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That appears to be the smaller locker cover, like the one I have in my 1979 Montauk.

http://photobucket.com/gallery/user/l...jc=/?ref=1

I believe (but am not definitely sure) all the Montauks before 1983 had the smaller cover.

As for the dimensional difference you are measuring, I can't explain. You might contact the vendors directly for help.