904 vs 727?

Well I've seen that but this one under the truck don't have that. The 904 in my 88 d 100 did.
According to the PK number on the one in the truck says it was used 81-85, part of the number is also a date code and I remember at the time I wrote it down and looked it up it was well in range of the truck build date, leading me to believe it is original one to the truck.