Not odd, most windlass manufacturers recommend, G-43 chain, a higher grade than the G-3 you have. It is likely either 5/16ths or 3/8ths. Sometimes a BBB type chain is used, of similar size.

If you can take a piece of the old chain with you when you go to replace it...if you can't do that and need to order via phone/mail then take some measurements and have your retailer figure out what size you have.