Yes it is still available.
Thanks to all the knowledgeable members out there, Jerky et. al.,
Taking a couple of pictures, it does indeed look like the rear cable guide has been carefully taken off - and I mean, carefully - you need laser interferometry eyes (or a good knowledge of this exact frame!)..
So, you are limited to the following brakes - V brakes (of course), OR you can go period correct with some yank type Graftons or Pauls, which are essentially V brakes before Shimano nicked them and popularised them later on.
You could also fit Maguras of course, it has brake bosses, and so there is no reason why you couldnt fit them.
Closer pictures of the shapely rear of the frame.. you have to look very closely for any evidence of a rear brake hanger. No reason why you cannot fit another one of course if that is your wish. Or even some other engineering solution of course.
Or just Maguras, with a "brake booster"..