Generally, I would have thought that if Ikelite can re-use an existing housing for a newer model of the camera, they would, and there are plenty of examples of this across Ikelite’s housing range. They would only go through the effort of configuring a new housing setup if they really had to - otherwise it is simply less profitable for them.
Conceivably, if the housing back from the housing for the E520 fits into the housing for the E500, then an option might be to try to obtain just the housing back for the E520. I assume your “The housing releases are .10 apart” comment relates to this concept. .10 what though? millimetres, centimetres, inches? If its inches then the housing releases probably won’t mate anyway, irrespective of anything else. If its centimetres then the releases would probably mate up ok.
Other reasons for the housings being incompatible would be control placement for controls that are operated through the main part of the housing rather than the housing back. If these have moved between the E500 and E520, then the E500 housing is never going to work with the E520 unless you can do without the affected controls.
The alternative I suppose, is to try to source a used E500 (ebay?) rather than a new E520, then your housing will be fine.
Sorry about your flood - that’s a major pain.
