Update #1 - You. Guys. Rock.
I had some time to take the D54 out today and shoot a little bit since my apprehension level was reduced significantly due to the responses that this unknown screw wasn't part of the trigger group and most probably an external screw filling a threaded void to mount a rear peep sight. So..... I took it out and setup a short 9 yard or so muzzle to target distance and sent some lead.
I just wanted to get a feel for what this beast was like. Using a large shooting bag as a platform, I had two pellet choices to use. Both tins were from about 1980 and included a HOBBY of unknown weight and a Benjamin HC also of unknown weight. I figured with the scope setup and an approximate 10m distance, this would be a good first outing for me.
Starting with the HOBBY pellet in the top left target, I squeezed off five shots. The first three I was quite pleased with and then two walked into near the same hole away from the first three. Hmmm. OK. At that close distance I certainly didn't feel I pulled them at all, but the target is what the target is.
Second round was using the Benjamin HC (High Compression as the label says). Man did they hit a different spot on the target but I wasn't so much concerned about that as I was just seeing what type of grouping I could get. Second shot walked up. Third shot walked up. Fourth shot walked up. I know I'm rusty, but this wasn't adding up in my head. After that fourth shot I thought the scope was closer to my eyeball and sure enough, the rear scope mount was 75% off the scope rail base! You guys weren't kidding when you said these D54's were tough beasts.
So I went ahead and slid the scope backwards and off the scope rail to continue shooting with the open sights.
Third round was the same Benjamin HC pellets but with the open sights factory to the rifle. I was kind of pleased with those although not near as 'tight' as the scoped HOBBY pellets.
The fourth and final round was the HOBBY pellets and still with the open sights.
Couple of observations.
1) This thing is a beast! Man it spits those .177 pills out quickly.
2) I had a blast with the first round and into the second inspite of not knowing why the spread kept opening up
3) I need to get a better mounting system because this could be a hoot at longer distances and the scope
When I was done shooting and since I had the scope base off the gun, I pulled out the unknown screw which was the basis of this thread. Sure enough, that's exactly where everyone said it should go! Threads were the same and the looks were the same. I can't express enough thanks for everyone who helped solve this part of the puzzle! (i still have zero idea where the screw was hiding the whole time. it certainly wasn't under the scope base)
Next up will be a different base to hold the scope in addition to getting the cocking lever to stay closed.
Thank you all thus far!