I came across this thread and also have a Ruger Mark I. How do you tell the difference between the t-515 and the t-514 and so forth. mine has adjustable rear site and what looks to be a 5.5 inch barrel. and is marked Mark I on the reciever. I to also just recently joined.
Been there done that. I didnt have the skills to fix it so I took it to gunsmith. Long storey short I know where I made my mistake and now can assemble the fun perfectly. Only cost me $10 to have smith look at it.