Koopyetz pretty much nailed it. The first concave back OBHs were minted with a specific die on which the worker has straight overalls, without any evidence of a belt. All subsequent issues had the belt on.
If you read some of the other threads and posts, you'll see that PMD's classification is now outdated. I'd suggest that you use Mondvor's one instead which is up to date and much more detailed. Even a non Russian speaker should be able to figure it out.
Yeah, something benign like Marc suggests, or take it to a jewller and have it professionally repaired.
"patina imparts character" - PJS
A wise man said: 'you should not pay a premium for what you want beyond your comfort level, and always ensure there is a mutually agreeable return policy.'