I found a used timer, very similar to Bruce's, at a local photo shop. It seemed to operate well enough, so I bought it to use with my Mamiya C330F. On first use, I, like Ari, thought it didn't have enough force to push the shutter button. Instant buyer's remorse. But, after playing around with it for a while, I discovered that it has a force adjustment - actually a plunger depth adjustment. It works by turning the outer plunger barrel, which is threaded onto the inner plunger barrel (the tube attached to the timer body), and locking it in place with the jam nut. Once I figured this out, and adjusted it correctly, it worked like a charm! The C330 shutter release takes quite a bit (comparatively) of force, but to use this timer on a sensitive, low-force release, it would have to be dialed back. Shouldn't be a big deal, though.
The proper adjustment of these little clockwork timers may be immediately obvious to some, but it wasn't for me, and probably not for many others. Ari, I don't know if the type you had gotten from Calumet was adjustable. They don't seem to carry them any more. But, Bruce is correct - there are a lot of them on eBay. Many of them seem to be dedicated use, for old Russian or German cameras. And, I only have experience with the one I bought. Every one of them may be adjustable for all I know.
Thomas
