There should be an indent at every speed. You are right, don't force it!
It's not a variable speed shutter, in that it is not made to run at speeds other than those indexed by speed numbers. IE, there is no proper speed between 1/25 and 1/50, etc.
Your's is an original Mat, made from 1957 to somewhere around 1971. They made a gob of them.
The thing mentioned about what not to do is, Never try to use the self timer when the flash sync is set to "M". There is a lockout to prevent it, built into the shutter, but I've fixed enough of them to know that people can overcome any idiot proofing that Copal built into the shutter on these cameras.
From having many of these shutters apart, I'd say that yours has a bent indexing pin on the speed gearset pallet wheel. The shutter speeds are set by a rotating cam on the front of the shutter. This cam has a number of slots cut in it to position a number of pins in the shutter works that give you the "B" setting, slow, medium and fast speeds. This little pin I'm talking about pushes the pallet wheel (kind of like an idler gear) away from the speed gearset to let the shutter run faster. It's bent, and as you try to change to speeds higher than 1/50 it's hitting against a flat spot in the speed setting cam that allows for slow speeds.
You can get to this pin easy enough if you are handy.
Take off the leatherette on the front of the camera.
Remove the plate that has the shutter button mounted in it. Watch for washers beneath it.
Remove the lens bezel (the part that has the speed and aperture wheels in it.) It has 5 screws around the perimeter, and then it will pull straight off. The flash sinc must be set at "M", and the self timer must be at it's "rest" position to clear the bezel.
Now you can see the actual shutter. Unscrew the front element, and beneath it will be a scalloped wheel, which is the threaded flange that holds the speed cam onto the shutter. It may have a set screw in one of the scallops. Turn it half a turn and then you can unscrew the scalloped wheel, pull off the thin plate beneath it, and you're looking at the speed cam. If you are looking at the camera with the bottom of the camera facing your chest, the pin I'm talking about will be at the 5 o'clock position. You can pull off the speed cam and very gently straighten the little pin.
>>You must support the arm that the pin is mounted in while you straighten the pin<<.
Also note that when you pull off the speed cam, the detent that indexes the shutter speeds will just be sitting there loose. Take it out so you don't loose it.
If you feel like going this far, but would like some pictures, write me and I'll email you some.