The 1975 rear window and its seal were changed by Volkswagen after chassis number 155-2012-217. If you have a 1975 then check your chassis number. The seal with suffix "E" is for the later cars.
Another way for 1975 VW Super Beetle convertible owners to tell the difference (if the rear window has been removed from the rear window frame)is to measure the height of the rear window glass (along the center line of the window). The early 1975 Beetle convertible rear window glass measures 11 1/8" top to bottom. The late 1975 VW Beetle convertible rear window glass measures 11 1/2" from top to bottom.
Some Information on Rear Window Selection
Before 1964 VW Beetle Convertible Rear Windows were small. If you desire originality then, by all means, stick to your window size, but bear in mind that replacement glass is going to be hard to find when needed. Later (and larger) rear window glass is generally available.
Any full convertible top restoration will allow the use of a rear window frame and a rear window from any year. In other words, you may choose a late 1975-1979 rear window framework and glass for any new top ! An advantage of this particular window arrangement is that its curvature is not as great as the glass in the 64-early 75 VW convertible. This makes the installation easier and "popout" is less of a problem. Also: mid75 to 79 convertible frames and glass are going to be available for as long as there are Beetle convertibles on the road !