Slight differences in bar and stem reach, crank length and lots of other things will change how even the same exact frame feel too you.
There really is no standards document that says a 50cm frame must be this top tube length and this seat tube angle or anything else. And on those old bikes, even if you go by the original manufacturers suggest for the size you should get, you don't know that someone in the many years since new didn't change the stem or bars to change the reach or changed cranks to change crank length.
So you really need to try them on. Make sure to ensure the saddle can be set to your height leaving enough post in the seat tube and that you will be in a comfortable position with your reach to where your hands will be on the bars.
If you are going to buy without being able to try, then just don't spend so much that you can't get another if you chose poorly.