Four minute Recording stili.

[edisonphonoworks]

I have a patent drawing that has a recoding stylus with a notch out of what looks like a door knob stylus.

[WDC]

Here are some closeups of the 4M recorder I have. With mine, there are some additional differences to a 2M recorder: First, the inner weight tube has the similar shape as those ones used in 2M recorders, but this one is made from lighter aluminum - likely to limit cutting depth.

Another difference I noticed is the much stiffer (and therefore thicker) mica diaphragm. I haven't measured it but I would say that it probably can be compared to a regular reproducer mica diaphragm. It does fit the lighter internal weight policy. A stiffer diaphragm would reduce the peaks to prevent echoing.

Btw. what appears to be a solder glob turned out to be a solder-covered dome. When I resealed the recorder with wax and rubber gasket, I could see the hollow backside of it.

So, with some little effort, it is very possible to modify a 2M recorder to make 4M recordings.