You didn't say what bike or sidecar you are setting up; so I'll address the Earles fork twin and Spezial (TR500) sidecar setup, because that's what I'm most familiar with, and hopefully some of it may be useful for your situation.
Attached is a simplistic sketch, showing the basic layout.
The illustrations in the Steib Service Instructions (thank you for the link Bill, my old copy is in German and seeing a good translation is well appreciated) depict the brake system as installed on a /3 sidecar setup.
On an Earles fork bike, the master cylinder is oriented with the fluid reservoir towards the front of the bike and the brake line attachment point towards the rear, which is the opposite of the /3 setup as shown in the illustrations, so note that difference depending on which model you're working with.
The TUV (Germany's DOT) codified stopping distances for motorcycles and motorcycles equipped with sidecars (in 1953, I believe) due to the large and growing numbers of bikes and sidecars in use at the time.
Steib's hybrid brake system was developed to be able to meet those requirements.
But by the late '50s sidecar popularity had peaked, with the postwar European economy improved enough that people were able to afford cars again, and the sidecar industry declined overnight.
Steib went from producing 50 sidecars a day, to 5, and then finally to none by the early '60s.
So that Steib pamphlet, the BMW Repair Manual, the BMW Instruction Manual, and the Service Bulletin may be the only literature available on this brake system.
It appears the master cylinder came with the opening on top or the side. Especially since they are so difficult to find, will either work, regardless of motorcycle or sidecar model?