I'll add my .02.
I rebuilt two XR5 RFVC engines not too long ago. One 83 and one 84. One was running and needed freshening up because it consumed some oil, the other was literally a parts bike with the engine in pieces, along with another 84 parts bike (enough parts to build the second bike).
First, the cam sprocket for these old RFVC only had two marks on them, like yours. I think the extra alignment mark came later.
When putting the top ends back on after the valve replacement, I tried to use the cam chains that I had on hand. All three of the used cam chains showed stretching just like yours.
I decided that I was only going to do the rebuild job one time (famous last words, I know
) so I bought two new chains from XRsOnly.
Both new chains lined up the marks perfectly upon installation.
The bikes do perform well after the rebuilds, which is what I was after.