That's a pressed steel cylinder. Neck threads are 7/8-14 UNF. They are mostly only available on 3500 PSI cylinders. Yoke regulators aren't rated for that pressure, in most cases.
Free stuff is free because it isn't worth the time or effort to deal with it. You've sanded the cylinder which goes against every cylinder rule, so now your Fill Station Operator will hassle you when you go to get it filled. You don't have a regulator that will connect to it, so now you have to upgrade your regulator at close to the same price that you would pay for a good used cylinder.
But you showed us. You came here asking for advice and everyone said to not use the cylinder, but you showed us.
Of course, you can spray paint it with a good zinc paint that you can get at Ace Hardware, but it will rust anyway. It will never be as good as a good used tank that will meet your needs with the gear you currently have.