I have seen some of these revolvers that have been neglected. The button and pin have been known to get gummed up, or slightly rusted inside. Before forcing anything, I suggest soaking it with some penetrating oil in n the button and pin. slowly start twisting and working the pin and button around. It should work loose eventually.
p.s. I think the earlier models did not have the release button on the side of the frame. But, just a ball detent on the inside, sort of like the ball that hold a socket on a ratchet. You just pull the pin straight out. Once again. soak in penetrating oil brfore forcing it out!
Good luck!
Jim