More pictures! I've seen this picture in several threads now, and it is impossible to tell what you might need for a chain. It doesn't show any of the details you would need to see to guess.
I see that it has a rear caliper brake. Why? There is no derailleur visible. Is there a 3 speed hub? I don't see a shifter or any cables other than the brake. Is it a 2 speed kickback or a coaster brake and they just put 2 brakes on the same wheel?
Guessing time I guess.... There were only 2 kinds of chain in common use in the 60s or 70s when that bike was probably built Both are still readily available:
A) 3/32 x 1/2 pitch for derailleur equipped bikes (5 speeds and 10 speeds that seemed to be everywhere in the 70s)
B) 1/8 x 1/2 pitch for coaster brakes and almost everything else. This is most likely what you need.
If it has a coaster brake in back (Bendix, Komet, New Departure etc) or a kickback (Bendix Automatic etc.) then you need "B". In fact for almost anything you would need "B". If there were pictures of the hub and the front chainwheel, someone here would have nailed it in about 10 seconds.
If, by some odd chance, it has a single speed freewheel, then I just don't know. You will have to measure the thickness of the sprocket on the freewheel, and the thickness of the chainwheel. Measure both with a caliper at the teeth, and if either is thicker than 3/32" ( .093 inches or 2.381 mm), then get "B". If they are both thinner then get "A".
Chains aren't sold to just fit. You need a chain tool to cut the chain to length. A cheap one will do. Buy a chain with a lot of links (108 or 112 or something) so you can't posssibly miss and get one too short. Loosen the rear axle, wrap the chain around and cut the chain so the axle runs in the slot mostly toward the front of the bike, but the tire isn't hitting anything. Then you will have room to move the axle back as the chain wears. You must cut the chain so that both ends are the "thin" link if you are using a master link to connect the chain (usually done with "B") or so that one end is thick and the other is thin if you are just pushing a pin back in with your chain tool (usually done with "A"). Half links are available for "B" in case the chain length just wont work out, but that probably would not have been done on a new bike.
