[GreenKeys] M15RO on with unusual cover...

[Jim Haynes]

I've seen the later bellcranks - made of thinner metal with a
rib pressed down the middle to stiffen it as much as if the metal
were thicker, but leaving it with lower mass.  I don't know how
essential that is to operation at 75 wpm though.

Some 15s were operated at 100 wpm during WW-II.  They required more
maintenance but the ability to handle more traffic was considered worth
it at the time.

jhhaynes at earthlink dot net