Capital asset fair value ??

PROVISION : The base year for computation of capital gain has been shifted from 1981 to 2001 with effect from A.Y. 2018-19. Thus if any capital asset ( acquired before April 1, 2001 ) is transfered then assessee has an option to take it's cost of acquisition either as fair market value as on April 1,2001 or it's actual cost.

Query :
if the actual cost was not known than what will be the fair market value ??