The Cheka thing is a total blind alley.
Lots of people wore black leather, because it was immensely practical. It was the Cheka "uniform" only because they had access to all the best stuff. If they wore long ones, it was because they were the best for town use. These guys are in the field.
There were no "Cheka" soldiers in the RCW as far as I can work out, they only had ordinary units attached to them. Though there might be some armed Cheka groups, in that they went in armed, they're unlikely to walk around with cavalry sabres. Nor would they be out in the field very much.
The "Red Guard" is also a red herring. It was a generic term for Bolsheviks, just as "White Guard" was for their opponents.
They are cavalry obviously. Unlikely to be Cossacks, as they are wearing spurs and the rifles are slung the wrong way.
Given how well they are dressed, I'd guess something like junior officers in 1st Horse Army, or ChON, as suggested above.