It was realised that the ends of the mine were not covered by the liquid filled pressure pad and that it was quite possible for a tank to harmlessly run over either end without damage.
Originally, the pressure pads were not extended to the end due to potential damage during storage and handling, also possible problems with the mechanical mine layer.
It became obvious during trials that had the pressure pads extended to the end, even if a tank ran over the end and the body of the mine was not under the tank, it was at least capable of blowing the track off.
Retrospectively, a mechanical and an electrical fuze, both known as FWAM (Full Width Attack Mine) fuzes were developed to overcome this

The purple bands indicate that it is 'experimental' and probably part of a trial testing different aspects of the mine.