I have 2 MDC LI-ION and 1 E2 scout head all purchased in 2013. All three have the brass ring, the 2 MDC have bare threads, E2 finished threads. No contact issues with any.
The 1X123 twist has a M31.
The 2x123 twist has a M61LL.
Both look to be neutral. Think I got the hosts about eight years ago but play a bit with the drop-ins.
Need to check on the MD2s, once i take them apart.. lol..
I love to collect the lights but I have not really learned much about them...