I can confirm that the "GRP. 7.504" identifies "BUSHING, Rear spring eye" in Chevrolet Master Parts Books from many year groups. Unfortunately the particular part number "509122" does not show up in any Chevrolet Master Parts Book/Price List that I have access to.
My best SWAG is these are for another GM product line Either Buick, Cadillac, Oldsmobile or Pontiac as I think they would have used the same Parts Group numbering system as the Chevrolets.
GMC used Parts Group# 15.0355 (at least from the early 50's to 60's) for the rear spring eye shackle kit from what I can see in my resources, so these wouldn't have been GMC items IMO.
