That code doesn't look right. The production code is in the space under "model" on the data plate. It should have a letter instead of the number 1 as the last digit. It should be an A,B, or C.
Assuming the 1 is wrong the rest would mean...
8=1978
H= August
09=9th day of the month
A,B,C indicate 1st,2nd,or 3rd shift.
Hydraulic pumps are easy to change on the 555. Pull the grille. The pump is right there. There are two nuts and bolts mounting it. Remove the hoses, and slide it out of the driveshaft yoke. If you have to replace the pump it would be a good time to check the U-joints in the driveshaft for the pump. There's no good way to lube the back one on the older machines.