Keg post question

Hello all. I just got an old Firestone 2 1/2 gallon corny keg. This is my first attempt at anything with kegging so please forgive my ignorance on part names. Anyways I took it apart just to see what everything looks like and how I would have to clean everything. Long story short here. I cannot, for the life of me, screw the gas in post back on. I am 100% positive it is the gas in post. Both posts are different sizes and do not fit on the other one. What I see is the post, a ball lock post, and inside is the poppet and a round plastic thing which appears to slide into the post. The plastic part appears to have some spring to it once it sit correctly in the post, having something to do with the poppet. I have tried for over two hours now to get this thing to screw back on but it simply will not catch at all. I have even taken it to my local home brew store and they are stumped as well. It cannot possibly be this difficult. It is essentially a nut going on a screw. But it will not catch the thread at all. So I took the plastic piece out and then it screws on perfectly however the poppet has zero tension in at and it does not spring back into place when depressed. This is freakin crazy. The post should just simply screw right back on. Does anyone have any idea about this? Is there some trick I am missing or a certain way this has to go back on? It cannot be this difficult. Any help is greatly appreciated.

Have you tried to screw it on the other side? I know that one post will seem to screw onto both nipples but there is one post that will only fit onto one. Same with the disconnects. I just started kegging too so I can’t remember which one fits on both and which one will only fits to its home.