Vintage 1975 could be a 1-66 or 1-48. There were also a rare 1:32 around.

Take a jag and put a snug fitting patch on the barrel. Push that to the bottom of the barrel. Your barrel is probably a 28 inch barrel. If it is longer then it would almost certain be the 1-66 twist. Take some white out and mark the barrel and the ramrod together. Now mark the barrel 1/4 clockwise on the barrel from the first mark.

Now carefully pull the ramrod up letting it naturally rotate with the twist of the barrel. When the mark on the ramrod reaches the second mark on the barrel, measure the distance from the dot on the ramrod to the muzzle. Let us say for instance the distance is near 12 inches. If we divide 48 by 4 we get 12. So in that barrel the projectile would have made a quarter revolution in a 1:48 twist barrel in 12 inches. If it were a 1:66 twist thw distance would be more like 16.5 inches between the dot on the ramrod and the muzzle. A 28" inline rifle with a 1:28 twist, the projectile should make a complete revolution before it exits the end of the barrel. To the quarter mark it should move about seven inches...

This is not exact, but it will give you an idea of the twist...