Suggestions about the date code?

[72timemachine]

From what I have read over at the Novak website, this manual sm465 transmission, along with the information I took off the side of the case was produced from 1968 to 1991. While I took it out of a 1969 c/60 dump truck, it appears as though it is not original to the truck.

D254
GM
6273212

I also read, from a different site that the original parts should also be within a 10 year span of the vehicle production date. So does this date code read: April 25th, 1974, or 1984? Or is that just it, given that this transmission was not made in 1964 or 1994.

Thanks for any help.