They used T51s on many cars from Mercedes to Saab, VW and even Fords.
Here are some pics of GT boosters that might help:
ATE Brake Booster GT.2 - Opel Photo Gallery
ATE Brake Booster GT.1 - Opel Photo Gallery
http://www.opelgt.com/forums/attachm...-mc-assy-1.jpg
I noticed the vacuum port on your part is lower left of the MC while the old part pic (and the vac port on my GT) is upper left of the MC. Hard to compare the length of the input shaft with the boot and hex union on the old part. Here's a pic of a nice one
Cadinum Plated Brake Booster pictures from cars photos on webshots
I am no GT expert but on the surface it may not be the right application but you can measure the depth and bolt spacing because it might also bolt right up and work with a small mod or two.
Jim