it is 2 layers of treated duck cloth.tuff like denim.
inner liner is same as material on an ironing board
i guess it is an attempt to keep as much oil as possible off the track. i suppose indy thinks it is better to have something than nothing. i would say it would hold most of the oil if a rod popped. most bigblocks i've seen go, due to the extended sides of the block extending below the crank centerline, do a pretty good job of containing the parts.