Red plates are definitelly available in Germany for manufacturers and dealers but I think they are not cheap. If factory wants to keep large fleet of cars themselves I think its cheaper for them to put them in normal plates than use only reds if they want to keep many car on the road simultaneusly. Almost all coutries have transfer and dealer plates avaiiable but authorities try to keep number of these down as much as possible. It could have been more relaxed in Germany back then.

 

My '78 euro #975 came with rub strips when new.