I have the same radio, unfortunately not working ...
Barry, what makes you think that there were no radios factory installed??????
Of course there were several radios factory options!
Just look at all the price lists I posted recently
just a small sample:
M095 - Blaupunkt Wolfsburg
M096 - Blaupunkt Braunschweig
M097 - Blaupunkt Emden
--- these were 1971 Radios for the /4
M450 - Blaupunkt Boston
M452 - Blaupunkt Frankfurt
M453 - Blaupunkt Frankfurt US
M454 - Blaupunkt Köln
M455 - Blaupunkt New York
M459 - Becker Grand Prix
M460 - Becker Grand Prix US
--- these were the 1971 Radios for the /6
Gustl