I dont know that there is a "proper" sling for the 500A TP8. I suppose you could use a GI websling? I dunno how far back the 500 variant goes.
The gun is a 500, (A = 12 ga.) and the TP8 (Tactical Police 8 shot I believe) refers to the barrel and magazine configuration, here its a 20" barrel with full length magazine and 8 round capacity. This would be like the predecessor to the 500A Persuader but with all wood furniture,
Is it a 70's model?
So, being that its kinda like a police/duty/riot gun... I dont really know what sling would have been used, but I imagine any sort of duty sling would be appropriate.
Glad to have you! Post some pics of that 500 if youre able!