Brown marks in spuds

[Medbury Gardens]

This potato was given to me as part of a collection from a old gardener who had advertised for someone to past it on,Its known as 'Stan Cloak',red skin with yellow eyes,inside is yellow and is a excellent baker/chipper.But in the 15 years i have grown it,it has all ways had these brown marks within the older spuds,not in the young tubulars.Its not rot as now 4 months after harvest the markings have not lead to any deterioration in storage.
Does anyone know what it is ??
Its the end of the road for Stan Cloak, i`m not going to grow it again, as i have been given 'Heather'to try this season as well i want to bulk up on 'mystery'(thanks for the name Tom)

Richard