Jefferson State AR15 Adaptor install question

Good evening all,

I am new to the airgun world and just bought my first pcp air gun. I decided on a a marauder pistol. I purchased a jefferson state air rifle ar15 adaptor and I'm having issues installing it. It's my understanding that it's supposed to replace the rear plug on the reservoir tube, the one that keeps the hammer spring in place. I went ahead and removed the screw on the top of the plug, as well as the top two screws. Once I got the adaptor in, I realized that it wouldnt keep the hammer spring in place. Am I missing something? Any advice would be greatly appreciated.