Mike, my buddy who is the OO of the 71 bahia/black 914/4 had the choice of mono or stereo in many brands from the dealer, as well as those with 8-tracks, like the Motorola AM-FM-8-Track that was in my 73 2L when I got it from the OO back in `75.
Basically you could get anything you wanted from the dealers, & I think Jeff Bowlsby's classic 914 website's options listing gives at least one mono from the factory, although most were just shipped "radio ready" (i.e.: with speaker boxes & antenna for radio install), for the dealer added options route.
While some may have been squeezed pricewise getting their 914s new back in the day (I know I was a 18 months out of college) & chose the mono, & IIRC I would think that they could get a non-Blaupunkt/etc. (non-German) radio or player with stereo to take advantage of the 2 speakers/boxes - for about the same price as a new Blau. mono.
That's what I personally recall, since I myself did look at some new 75 & 76 2Ls back then too, & considered the dealer radio options, since I could finance the new car 100% for 5 years - vs. 80% for 3 years - with about the same monthly payment. So I looked at both new & used, but went with the 73 2L due to it's better power, vs. the "smogged" & crapalytic converter equipped 75-76 2L.
Bottom line, I don't think the other person was correct - & certainly NOT for the majority of 914s with dealer radios.
Happy Holidays!
Tom
///////