As noted above, your car was probably originally equipped with DOT 5, which many of us prefer for cars that are not driven regularly. But anything could have been added over the past 50-60 years. The easiest way to tell what's in the car now is to suck a little out of the master cylinder with a turkey baster and put in into a clear glass jar. Add a similar amount of DOT 3 (it's cheap and available everywhere) and shake it up. If the car now has DOT 3 or 4 in it, the two fluids will mix completely. If the car has DOT 5, the mixture will turn milky and eventually separate into two layers.
If you want to change to a different type, you really should flush the system completely with a good solvent (alcohol or acetone) and replace all rubber components, like hoses and M/C and wheel cylinder cups. Replacing hoses is always a good idea in any case. Good luck..