RBT mounts are available in five sizes. Below left you see illustrated a typical RBT mount. This one has the proportions of the 7 perforation 28 x 23mm aperture size.
Currently there are also 21.5 x 23mm (Realist), 31.5 x 23mm (RBT Full Frame), 33 x 23mm (Full Frame) and 21 x 16mm (Nimslo) aperture sizes.
Firm fixing and adjustment is provided for 
b
b
the use of plastic "pin-bars" which fit nicely into the top and bottom recessed rails. The film chips fit over accurate "pins" on the pin-bars. There are marks on one side of the pin-bars which permit them to be placed as illustrated to achieve a vertical variation of from zero (1) to 0.4mm (5) in 0.1mm increments. (There are alternative pin bars available that allow adjustments in approximately 0.2mm increments to 0.7mm).
Accurate baseline or vertical registration is achieved by working to a proven jig / grid mounting system or by trial and error, checking progress in your stereoscope (viewer). Freeviewing can also confirm baseline accuracy, particularly if enlarging lenses are used. The most useful non-projection jigs have a light-box under and a pair of lenses over.
When it comes to adjusting for horizontal position, move the chips on their pin-bars left/right according to your already proven and preferred method, being careful not to violate the stereo window. As soon as you
are satisfied with the adjustment of the film chips, just press the other half of the mount into place on its locating pins. The mount can be easily reopened at any time.