Hi Gordon
SCR = Signal Corps Radio ?
This link has some info
http://radionerds.com/index.php/SCR-187
BC = Basic component of SCR
The BC 191 Is the transmitter part of SCR 187, this transmitter is also a part of other SCR sets like like SCR 193 as used in 1/2 ton Dodge command cars .
The set appears to be intended for air to ground liason use, and it was fitted in Piper Cubs and similar aeroplanes with a 12V System ?
The BC 191 is a 12V transmitter .