These are HD mounts that were offered through dealers and FLAPS as a replacement part. I believe it is mentioned in the Master Parts Catalogs but not in the data books. I have also seen them as NOS items offered occassionally on eBay. The reason they were offered is for the reasons you see in the weld repairs on the crossmember I have not seen many trucks (47-53) that did not show evidence of stress or cracking in this area. The new 54 style is completely different for a reason and that is the original 47-53 style were sometimes a warranty/service problem. These may also be mentioned in one of the 49-53 service bulletins. I think recall seeing them but don't know which series.