Hi Jerry,
I have seen pictures of type also, but have never actually seen one in person. Not sure either if they were unique to a specific era, model, displacement, etc.
I was tempted to think that perhaps they came later than the cast/forged rockers (maybe as a cost savings?), but if you look at the rocker arm shaft support brackets in that listing, they are cast aluminum. I believe those came first, followed later by the stamped steel brackets. Although its also very possible that assembly was put together from multiple sources some time it its history.
If any GMC guru on this forum is reading this, perhaps they can shed some light on this.
Best Regards...
