The truck's Main Trans Case does not have a fluid filler smoke stack cast into the case LIKE THIS 1954 Chevrolet MAIN CASE.
It must of been ether 1958 or 1959 when GM changed the HM main case and stopped casting the smoke stack into the case.
1957 CHEVROLET TRUCK OPERATOR'S MANUAL, HM fluid Check Smoke Stack
1958 CHEVROLET TRUCK OPERATOR'S MANUAL, HM fluid Check (Not Available) Smoke Stack or Pan fed ??
1959 CHEVROLET TRUCK OPERATOR'S MANUAL, HM fluid Check Pan fed

This eBay auction Shows the smoke stack dip stack used from 54-58. part# 8605800
So the transmission in the truck in question is probably a 1959 model year transmission.
1959 was probably the 1st year GM started using a pan fed tube on the H-M.

Anyone know where there is a online 1958 CHEVROLET TRUCK OPERATOR'S MANUAL?

1954 Chevrolet 3105 Deluxe Panel Truck
w/Factory HYDRA-MATIC