Thank you for posting the query about WM-D6C serial numbers, because when I went to look for mine, I discovered that I'd stupidly left batteries in it for I don't know how long. Fortunately they hadn't leaked.
Anyway, the serial number here is 21563, and I've had this one since the time of the third Punic War. (Seriously, I might actually have bought it in the 1970s, if I recall correctly that the model existed then.)
In my opinion there's what people call "no comparison" between the WM-D6C and the TCD-5M; the TCD-5M has a much better transport with lower scrape flutter and modulation noise, plus more reliable connectors, metering that's actually usable for setting and observing levels, and a built-in limiter. The TCD-5M is more expensive than one might feel it should be, but it is the only fully portable cassette recorder in its performance class. If only it had three heads and adjustable bias ... but then it would be a $1000 recorder, and the time window for that passed 25 years ago.
--best regards