I believe all 4 way tool posts have the pin mentioned. As said it's used for quick indexing. On my lathe the pin & spring goes in a hole that's on my coumpound slide towards the front instead of the t-nut like shown in the diagram.
I'm not sure if that's what your pin is though. The pin usually has a bevel on one end so you can turn the 4-way TP to index but it won't turn back the other direction. Your pin doesn't look to have a bevel on one end. At least that is how mine pin was as well as for my mini lathe.
What do you mean by inside the compound, like it was actually inside the compound slide or under the compound assy?