Need to drop first two characters from url

My blog has been around since 2005, and at this point I’m having an issue with permalinks. At some point, I changed them – probably a few years ago, and now I’m noticing a ton of 404 errors.

I’m currently using %postname%, but there are lots of links out there that, for an unknown reason, didn’t have the %monthnum% from the old /%year%/%monthnum%/%postname%/ stripped off when the permalinks were changed.

Now I’ve either got to do hundreds of redirects, or come up with a way to strip those old month digits off with .htaccess if they are present. If anyone could tell me how to do this, I would be very appreciative.