1861 vs 861 casebacks

Are there any actual differences between these two? I ordered a display back from http://www.customwatchconcepts.com/ and in the last second I clicked something wrong and ended up ordering the 861 back. My understanding were that they were the exact same, but there's a difference for 1861 and 861 (different part numbers on the site). I tried to correct the order, but they're not quite as good at responding to questions as they are at making awesome casebacks (no response yet even though I have the back already installed...).

Gasket seals fine, and the threading seems to be the same, so If there's any difference it must be in the movement ring, but it seems to fit.

Your case back number will be more relevant than the movement number. He lists the compatible cases with each case back

Mine is a 3570.50, the 1861 back they sell fits (according to them): 3570.50, 311.30.42.30.01.005, 311.33.42.30.01.001, 145.0022, 345.0022
The 861 back they sell fits (according to them): 145.012, 145.022, 3590.50, 145.0012

I have it mounted on the watch, seems to fit fine, without any issues, so I was wondering what the difference in general might be between these as I always thought the backs could all be swapped

Edit: I should also add I've seen multiple listings of these casebacks that indicate "fits both 1861 and 861" which is why I'm asking

As far as I know, the only difference between 861 and 1861 is that it is rhodium plated.