The business of the multiplication factors that people have to apply to lenses to figure out what their focal length means has always been a problem for customers and staff alike. Not that we are primitive people - well, I am, but that is beside the point...People can do and do do the mathematics involved...but it really seems to be doodoo.
The whole comes back to trying to get the idea of the angle of view that each lens encompasses into the visual mind. It has not been helped by the use of the terms 'standard' 'wideangle', and 'tele'. Standard for one person's vision may be entirely different from a technical definition of the diagonal of the sensor size and sometimes you really don't want to go down the road of explanations...
Had the manufacturers adopted a simpler scientific system of designating what you see it might have been easier - as it is many of the lens charts and promotional material they send out do contain the information but it is always secondary to the focal length and the multiplication factor. The scientific number would be the angle of view - it can be expressed in degrees.
Thus if you have a 24mm x 36mm sensor on your Flapoflex camera and you put a lens with a 50mm focal length onto it you see a certain number of degrees as the diagonal angle of view. If you put that same lens onto the 18mm x 24mm sensor version of the Flapoflex the diagonal angle of view becomes smaller. The pocket Flapoflex even smaller. No mathematics - just numbers.
Won't happen commercially - but if you care to look at some lens tables you can work it out for yourselves and then use it for all your own lenses. Might help you to navigate easier.