As johnny indicates, nothing should be assumed—especially when it comes to old motos—although assumption seems to be the first line of reasoning for novices. Parts are swapped regularly among these motos when needed for convenience, price or style. Parts fiches like MaxBMW's have photos with measurements that show the differences among these handlebars cables and brake lines.
When it comes to the claims of aftermarket vendors regarding fit, query should be made to them to determine what they're measuring—hose length only or hose length including fittings then you should measure from the same points to confirm fitment of their parts.
Thanks for the heads up on the fiche for classic BMWs at MaxBMWs. Assumptions -- so begins my lessons for the day.
I measured the length of the front brake line on my K75 and it's 940mm in length tip to tip. Investigating the fiche at MaxBMWs shows that part to be 810mm in length. Just got off the phone with Spiegler and their stock from line for a '92 K75 is 910mm. So the bars on my bike are obviously higher and/or swept back a bit. The Spiegler rep concurred. So much for assumptions.
Spiegler can make the custom length at no additional cost. Now I have to check the length of the rear line and then get out my credit card.
Here's a couple of pictures. The first indicating why I didn't want to get into this right away. And the second showing my handlebars. Perhaps someone can illuminate me on whether they are different than stock. Whatever they are, they are comfortable for me and keep me in a nice, sitting on a toilet, position.