I don't think these 990XXX Bearcats show up very often. Recall sometime in the early '70's, I saw one. Back then, I was corresponding with Jeff Munnell and told him about it. Jeff did not believe I had read the digits correctly. Used to be a joke that if Jeff did not have one, then it did not exist!
Yes...Ruger was having a lot of trouble with their serial numbers at the time. A batch of 88xxxx range 10/22's were also made.
Here's a 99xxxx range Bearcat that had some major re-work done at the factory....I only know of two of these re-worked 99xxxx guns so far...by the way...Munnell used to own the other one also, number 990290 which last I knew, JD had.
If you look closely you can see what looks like the trace of another digit under the two. Before we whited in the serial number you could see traces of other digits as well.
and a D98xxx range gun that had the same treatment, the only one I've seen...
Note the earlier "D" to the north of the final "D" in this serial number.
Both guns are also without the SR in the eagle on the barrel which means the were finally completed several months after they should have been, at a time after the barrel rollmark had changed.
Both serial numbers are also very heavily stamped, the easiest way you can tell this variation...the result of a much thinner portion of the serial number side of the frame where most of the remnants of the earlier serial number were ground away.