Wasn't the tape also 4-track, 1/2" width?
Bernie,
The tape was 1/4" wide 4 track, similar to what you would use in a TEAC 3340 (remember those!). The tape transport was modified as per the 1/4" conversion kit that was available for the original M400...
1/2" tape would be too wide, that is one of the reasons that standard Mellotrons (except the M300) use 3/8" tapes.
All the best,
Norm