F412 and maybe F411?
Indeed, the F412 bulkhead is doubled up. One flange faces forward, and the other aft. You will need 2. As recommended previously, check the next bulkhead forward as well. (F411). The 411 is also 2 bulkheads back to back. I have made this repair for another guy, and it's not that bad. If you just had the mount attached to the bulkheads with clecos, and not bolts, there is a good chance that you elongated the holes on the F411 bulkhead as well. (It tries to pull the clecos through) Look closely.
Not a huge deal, you can fix it.
GB