That serial comes up as a 1982 150 ELPT, originally sold to a fellow in PA in 1983.
Your pump is mounted inside the boat ? If so, check out p/n 99186T1 and see if it looks like what you've got. That number supersedes to 826729A10, which, given the price, looks to be a kit to convert it to the external motor.
Merc shows only the whole pump as an assy, with no parts available except a seal kit and the brush springs (no brush kit)
The equivalent Sierra p/n is 18-6273-1
If that's the right pump motor, you should have a 3-ram though...?
Unfortunately, I don't even have any junkers in the boneyard of that vintage to put my hands on. They don't last that long in the salt.
A pic would let us nail it down.