...I wonder whether....
Don't we all?
I have a working theory on many such questions. Remember, the oldest of these cars will be 60 this coming year, and the newest of them will 52. Unless you personally bought your car new from a dealer, and stored it away in a bubble, well, anything is possible in such decades of time.
For the first few years of these [new] cars lives, they were most likely maintained and serviced by an authorized dealership. They only used--so we think--MB branded parts in their supply chain. But what about dealerships in distant locations? Maybe one in The Philippines, Central or South America, or Africa or someplace that may not have had a ready supply chain of factory parts? OK, just interjecting the possibilities.
Once that new car hit a certain age, many of them lost their luster and many became simply nice drivers. Dealer servicing may have changed to local independents, using the best parts they could find or the cheapest depending on the client. So maybe your fan belt was changed from MB to Continental and eventually to NAPA...when the independent went to change the wires, They used what they could get. Far too many mechanics in the interest of their client's pocketbook (invoice) will change what's necessary only when broken. How many times have we had new owners/members here look at their wires in puzzlement to see a mix of horizontal and vertical as well as Bosch, Beru and unnamed ends? Or a mix of actual wires? It would be nice to think that a wire problem somewhere in the car's life would cause the entire set to be changed but that wasn't always the case.
When my car was restored, I pulled what looks like the original wire set off, and replaced with something I could find. Don't know what it was, but I'm sure it wasn't correct. About 12 years ago, I had a set custom made for me from some firm whose name I have long forgotten in California. At the time it wasn't cheap, well over $100 when a cheap set was about ⅓ of that at any local parts store. But it featured metal Beru ends, Beru solid copper wire, and all vertical ends for the distributor. Somewhere in my parts bin is a nasty old distributor cap with the old wire set attached. Also all Beru, though I cannot say with certainty it's original as delivered. My uncle drove my car hard as a daily driver for 10 years in New York, for an additional six years as a daily driver. Most of its maintenance over the years was at HIS father's Taxi garage! So anything was possible.
In Classic Mercedes (UK) magazine that came to my mailbox yesterday, there's a nice story about the provenance of the Sir Stirling Moss 230SL. At the end of the story there's a line that says the car "was delivered from the factory with a Blaupunkt radio" which may be a typo, but if accurate flies in the face of everything we believed about radios and Mercedes-Benz. I've always been told that from the factory, it was only Becker.
I guess we can always keep on wondering about everything.