I have a set of unusual Shimano 600 cranks (photo attached). The chainring spider has three short arms. Can't find anything online about these. Can somebody provide some information? The date code is AD, which I interpret as 1976. Thanks!
The bolt circle diameter is about 95mm, with spare rings available down to 30t. I don't know of any compatible rings: the more common 3-arm cranks used 116mm bcd.