I looked in my '51 100 to 350 series and 55.2 100 to 450 series GMC Maintenance Manuals and found no mention of these gizmos. But, I did find them in the 39-54 GMC Master Parts Book.
I'm pretty sure that my '52 228 does not have them and I'm real sure my 302 head doesn't.
What year maintenance manual is your picture from?
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
