This UKC review seems to be the only place with any recognition of the 'granite 50'. Online shops only mention the 25 or 16, no sign of the 50. Not even the Pod website has it.
Lorraine McCall began her attempt today at a continuous, self-powered round of the Grahams, a four-month tour of upland Scotland combining walking and bikepacking. If successful, she'll be the first person that we know of to do it.