An interesting thing about Bosch pumps, they continued making them for a long time, and parts are still available. I don't have anything that says how the tractor could be ordered, but I know someone who has the Bosch refit kit for the D691 in a TD18. It is a package deal with all lines and miscellaneous hardware. It would be possible that the tractor could have been ordered with that pump but was most likely swapped in later.
The Bosch pump is a bit of a maintenance nightmare by comparison, but has one distinct advantage. It is better at delivering rated fuel over the entire RPM band. The IH pump drops off below 1000 RPM or thereabouts, meaning power loss. Almost no power available at idle. The original IH pump basically delivers max fuel only at max RPM. Since the governor is mechanical and has to stop delivery at high idle, it tapers off fuel as the peak is reached. For most IH pumps the spec is 11% above rated RPM. If you ever wondered why the high idle numbers are kinda weird, factor in the 11% offset and you will always get a nice round number for the "loaded" value. The later B pump corrected some of the low RPM delivery problem.
For IH pumps, factor in the rising rate fuel delivery in conjunction with the falling rate governor shut down and you get a humped curve along the RPM chart. In a way it was one of the problems with IH diesels. The perception was that they didn't "lug" well. IH didn't consider lugging a feature. (And rightfully so. Oil usage at the crankshaft is speed sensitive) In the crawlers it lead to running them at higher RPM all the time. I tend to do it myself. When revved they kick butt and give a sense of serious accomplishment, but it is hard on the drive train.
I have a doc from a briefing at a hotel somewhere, but it isn't dated. It describes the process they went through in developing the gas starts. Not sure where it is right now though. The pile o stuff has gotten a bit out of hand in recent times. I'll have to search for it.
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