The CW Key on the BC-451-A was never intended to be a CW Key like the J-37 or J-38.
On the B-17 the BC-451-A was mounted on the pilot's overhead console and the pilot would have to reach his hand up and hold it in by pushing forward. Not conducive to sending any messages other than perhaps SOS.
On the B24 (D Model) the BC-451A was located on the console between the pilots. You could certainly send a message using the built in CW key, but again, not conducive to any message other than perhaps SOS.
I do a lot of WW II Aviation reading and for years I've been trying to find at least one solid reference that a Command Set was used on CW during a mission. So far I haven't (still looking). I've noted passages which could indicate such, but nothing definitive.
Last year I attended an on-line CAF seminar by Tom Culbert author of the book “Pan Africa – Across the Sahara in 1941 with Pan Am” by Tom Culbert and Andy Dawson. A very good book about the USAAF contracting with Pan Am to improve the existing Imperial Airways Route across Africa in 1941.
I purchased the book and found the following on page 105:
“Some radio operators used SCR-274 Liaison Radios, that were installed in the aircraft to practice their “ham” radio skills. Using their personal call signs and jargon, they could shoot the breeze with their friends and colleagues from Liberia to Cairo. PAA_Africa received word that the British were very concerned about the strange transmissions near 2 megacycle frequency, and the practice was stopped”.
I subsequently emailed Mr Culbert with the following question:
“In the paragraph above, the SCR-274N were the Command radios, not the Liaison radios. Assuming you meant SCR-274N, might you remember if the hams used Voice or CW for their activities?”
Unfortunately, he responded “Sorry I can not add to your radio question. The info in the book was obtained from one or more of the PAA-Africa radio operators - who have all passed. We used their descriptions in the text.”
73 Mark K3MSB