that is the middle drive cover cap. mine fell off years ago, the rubber seal dries out. a new one wasn't very much, maybe $15 if i remember right. don't buy the "special tool", when you get the seal, take it to home depot and buy a rigid electrical conduit coupling, i believe it was 1 1/2", it fits perfectly and costs $3 or so. take the seal with you and you will see how it should fit. tap it gently in using the rigid coupling and a plastic dead blow hammer or something similair. make sure it goes in STRAIGHT and not CROOKED. if it comes out that easy i imagine your seal is dry, the new cap has a seal built on. there should not be much pressure at all on the oil in your crank case, but ANY pressure will make it fall out if the seal is dry. a new cap should fix your problem.
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