Unusual Shimano 600 crank, three arm?

drpaddle

Dirt Disciple
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!
 

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Many thanks for this very helpful information. I am always so very impressed by the expertise of the old guard on this site!
 
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