Not as bad as it seems. All sources agree 51-6528 (5870) became M-51. Indeed SOC 15Dec1970 (Dutch AF records confirm). And it did become N650. What happened afterwards I do not know.

Forget about the gateguard M-51. This is M-54 painted up as M-51 at Woensdrecht AFB. Different aircraft than the real M-51.

Thanks Eric.
The M-51/M-54 story was known to me.
Actually I took a picture on a basevisit at Woensdrecht AFB:
https://picasaweb.google.com/lh/photo/pHyZ7l0NkJdHAsjhGSNm5BuAThP9JQzlXWbvnXyQN5U?feat=directlink