I suspect you're out of luck, but I'd ask IMTRA.
North American High Test (G40) and Proof Coil (G30) chains have “nearly” the same dimensions (inside length & width), excepting wire diameter. There is also an NACM "Short Link" Chain.
Perhaps GMac can provide the dimensions for 5/16" ISO
"G3" chain (wire diameter, & inside length & width.
NACM Chain Specifications (see the tables for dimensions)
Lofrans Windlass Gypsy
(chain wheel) Chart:
The above chainwheels are designed to fit short link chain only; BBB, ISO spec, etc. to maximize the number of pockets and gripping. Chain is manufactured by many sources. It is critical to match the chain to the chainwheel, therefore please be careful when sizing. Imtra only offers short link chain, BBB and ISO Spec. Do not use NACM spec chain
as these have longer links and will not work in a windlass. Non-ISO chain sizes like NACM spec or miss-matching the chain and gypsy may allow the chain to skip in or jump from the gypsy pockets.
If you have an older Lofrans model, we are likely to have replacement wheels available.
Please contact Imtra by phone
or by email
for assistance sourcing the right wheel for your windlass.