It isn't a Mark 12 and I am uncertain what to mark to give it. The registration points to being a trials only MBT. signified by the letters SP (Special Projects) and never issued to any regiment if that is the case.
There doesn't seem to be a Light Source box next to the gunners sight? yet it does have a MRS shroud on the end of the barrel and the canvas cover on the turret for a .50 Cal ranging gun.
It has a strange antenna mount on the right side of the turret that clearly shows it never received a Clansman radio suite and indicative of an early mark, yet is fitted with a Mk.6 NBC pack.
Many bins are missing as well.
This may have been a vehicle from Shrivenham or Borden used for training purposes? Or one used for testing bridging and BAARV recovery.
I would imagine the Museum can answer the questions but the registration is not listed in Rob Griffin's book Chieftain which contains the most comprehensive listing of all Chieftains built and issued to user hands (regiments).
Keith.