GMC Master Parts Book lists part number for your '51 250 series truck's rear shocks as part number 5513639 with an asterisk that says "Determine if option required". Same book shows Frame Shock Absorber Bracket part number 3693689 with the same asterisk.
Looks like an option to me.
'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.
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