found a real deep crank pulley today, what is it ?

[R72K5]

pulled it off a 76 truck 350 engine, it makes fourth groove on crank balancer, its a single groove super deep add-on pulley, its 3.75" from backside all way out to front edge of groove, it sits inside of the common 70s main three groove pulley, i never knew 73-up had a deal like this, thought only 72 older p/s and a/c had add on crank pulley setup like this

39500026 it says on it, its painted equipment gray solid just like remanufactured engine block color is,

what pulley is this and how uncommon is it ?
ive never heard of or seen an addon crank pulley for 73-up before,

wierd,.

thanks