The 28 x 40 x 7 mm sizes seems right.
That matches the size I noted on the invoice from the NAPA store when I bought the lower seal for my Burman box.
The part # shown on the invoice is 10930, however I don't know if that # is Napa's or the manufacturer's . It's described as a "transeal".
Remember to clean up the shaft with some fine paper or emery cloth. If the lower bushing is worn (high mileage ?),
the new seal may not be 100% effective. I don't know how much trouble it is to change that bushing, but if there's still leakage with the new seal, I've read you can fill the box with a heavier weight oil or light grease.
I have a bunch of notes on rebuilding and adjusting burman boxes that I saved off the Alfa Digest. I can forward those along to anyone interested.