Because GM parts wiki only has two books for GMC, 1951 and 1955-1965. When you are looking for a GMC only part you need the parts books.
The 51 book shows the part (pressure relief valve) in Group 6.0180 as 2233181
The 55-65 book shows the part in Group 6.0180 and 6.2065 as 580500
Two different part numbers with no link to eachother.
So you would probably look for the part using P/N 2233181. But if you had the Master Part Book 39-54 or a more current 54 book, in Group 6.0180 580500 replaces 2233181now you have the link and two numbers to search for. Then go back to the 55-65 book and in Group 6.2065, 580500 replaces 580476. Now you have three 2233181,580500 and 580476.
Chevy did not use this part.
