There should be one on the port side and one on starboard .
If it is like the ones I took out of #45 last summer , the flange itself was thru bolted .

Get some P-B Blaster at the auto parts store and spray the threads several times for a couple of days if you can wait that long , it should turn off/out with little resistance .
You can turn the flange from the inside too , after you remove the bolts .
Then glass the holes .
The Blaster really helps , don't use WD40 .

Mike G