All types of UZRG fuzes function by the same mechanism and differ only in materials used and explosive content.
It replaced the Koveshnikov fuze which was more expensive to make.
Both fuzes lived long next to eachother during ww2.

All photos below thanks to Dr.Ruby.
These are all ww2 levers.

Safety lever markings WW2 :

Roly has found this text on the net, this might be it.
Who can confirm?

UZRG - with "closed top". Delay tube can be brass and from pressed powder iron.
UZRGM - with "open top". Delay tube can be brass and from pressed powder iron.
UZRGM-2 - with aluminium delay tube; handle have different length from nail hat to safety pin (almost 1mm) and holes for safety pin not match; handle can have big hole and mark UZRGM-2. In delay tube used different ignition composition.


There is also a post-war practice version of this fuze, which had a replaceable black-powder charge.
SEE THIS LINK