I think it's not a custom job. The front panel is original for shure. The S letter is just the same of other Drake letters and is just covered with red painting (in my radio color is a little faded) as for example R4-c CW...
there is also no sign of new soldering or mods.. this evening i had a mail of another ham who has another R4bs and he is still searching infos about this radio, but he told me that he bought the radio from the ham who has bought it new in the seventies ad he said it was a commercial version sold directly by drake reseller.
it's an interesting mystery!
here is the photo of my R4b on the right, and the R4bs on left they look the same except for the vfo.
http://boatanchors-net.googlegroups.com/web/drake00001.jpg?gda=VPhHCT8AAAAq4Jfr9NL35UoorHBy-WgBvGafycGwXk8TRk9v5maqmIR-EV-5a9R9atAb2A8HkiSsYm-9u3SNcX6DNnN64i2pthe serial number of the bs is 19744 B
