Congratulations on your excellent winning spin! This just shows how random a slot machine is - removing the DBV-200, cleaning it, then putting it back in the machine wouldn't have anything to do with getting those three 7's. But it's still a nice feeling when a spin like that happens.
What you found by removing the bottom plate and looking at the eprom and its sticker tells you what the actual software installed is, that old sticker on the top of the DBV is no longer valid. The software in your DBV-200 now is ver 2.60-04-03. (the JCM document you found and posted is for a different version (2.60-05), so that document doesn't apply).
There was one newer version of software that was released, it is called 2.61-04-03, dated August 2006. From reading the notes on its document (attached below) it looks like that update to the software had to do with improving recognition of $10 bills. So updating your DBV-200 to that software probably would not make your $20 acceptance any different. Hopefully a good cleaning will get it to work better with the $20 bills. You can also try other denominations, it should work with a variety of bills, although the latest $100 may not work since it got redesigned recently, and perhaps the latest purple $5 bill may not work either.
You could still do the update if you want to, since it is an eprom in a socket you'd just have to replace that eprom with an updated one.