Rare, following definition from The Oxford English Dictionary:-
adjective (rarer, rarest)
(of an event, situation, or condition) not occurring very often:
a rare genetic disorder
[with infinitive]:
it’s rare to see a house so little altered
(of a thing) not found in large numbers and so of interest or value:
one of Britain’s rarest birds, the honey buzzard
unusually good or remarkable:
he plays with rare sensitivity
So there is no reason why the Concorde mower should not be classed as rare.