These are very large 1620′ rolls of flat waxed nylon synthetic sinew. Enough for many many boats.
The white colour usually becomes somewhat translucent when in use. The high stretch factor of this material is desirable over comparative polyester products. It allows for tight but flexible running and static lashings. Use a flame to melt a small ball onto the bitter end to prevent slippage of knots. May be split down for model making purposes.