The iron ring (iron in real life .. resin in our modeling world) fits either the offset or centered Storm Drain System Covers. This is the mount for the Manhole cover
When printing on a MSLA printer (Masked Sterolithograhy Apparatus) the models are supported on scaffolding called supports. The slight texture where the supports touch the model (a few thousandths of a mm) can be left .. “as is” but I prefer to lightly sand that surface. If you look at the example here .. the ‘rusty’ one the Storm Drain Ring the the white one above it the Sanitary Sewer Ring. In the “real world” the top of the Sanitary Ring is the only thing most people ever see as this is the ring around the manhole cover we see in a road. The Storm Drain rings fits into the Storm Drain cover.
I simply sit the covers and the rings on an emery board .. making sure that the writing is up on the covers and that the little recess for that cover on the rings is also facing up .. them push them back and forth across the emery board a few times with the pad of your finger. That is all that is needed. Clean up, prime, paint and install.