Fox Collectors

Is there a list somewhere that show known aftermarket "upgraded" Fox guns?
I have seen 2 "X" grades without the rebates on the top and bottom tangs. Were these factory early "X" grades or did they all have the rebates?
I have seen some that were ethically called out as upgrades and others that were kind of "stealth" grades.
Seems like it would be a good list to be part of this site.

Most of the early XE-Grades, with the collector desired deep engraving, don't have rebates. In addition to the rebates the profiled breechballs and the lock-screw on the top tang screw seem to be really hit or miss.

Thanks

What was the approx. SN difference between the rebated frames??