Found this: http://windsurfingdirectaskevan.blog...er-i-have.html
Notice that the author interchanges the use of the US, A and E box, but I seem to recall that there were dimensional differences.
The “E” box MIGHT have been thinner, thus fins with “bumps” on them had to have the “bump” removed, otherwise it would not fit down into the box. I had to do this on a Bic Alto waveboard I once owned when I fitted a non-Bic fin. It is possible that the “E” box is used in “thin tail” boards where space is at a premium or custom boards, as you have mentioned.
“A” box and US boxes MIGHT have been the same (does the “A” stand for “American”?) and are deeper than “E” boxes.
Am very willing to be corrected on the above.
Just took some measurements on my 2009 JP RWR 82: depth of fin box, 26.5mm. Depth of JP fin head, 24.5 mm, so there is 2mm (approx 0.080") clearance between the top of the fin and the bottom of the fin box. There is no bump on the JP fin.
I have an MFC FreeWave 28 cm fin, with a bump at its front end which I just fitted into the JP RWW. It appears to me that the bump controls the depth when the front of the fin is fitted into the box and the screw is tightened. The bump appears to be superfluous, because the original JP fin does not have the bump, nor does it come anywhere near to contacting the box when the fin screw is tightened. NOTE: the MFC is sitting slightly above the fin box, so the bump has to be removed or slightly filed down.
It would be great if Bill Kline (gsport) or someone from Chinook, True Ames, Rainbow etc. could clarify/add to the above. You might care to post the query on the rec.windsurf forum (it would make a nice change from all of the clothing ads!).
Hope this helps.