it's a bracket for a smog pump, (not used) gm had a handy idea of using the same bracket on the 305 and 350 trucks, to save money. so it was the same, just alot of trucks, almost all of them came without the smog pump.
Not that Ive ever seen. I was looking at an 88 - 90 bracket a few weeks ago and it had nothing for a smog pump. It looks just like the SYTY. Just go to the junk yard and see.