Hi Tony & Clevo
I was thinking about the webbing in between the cab & frame . It could be a mixture of Stockholm tar & Linseed oil that the webbing was soaked into. I use Stockholm tar as a thread anti seize on track bolts on earth moving equipment it is brilliant if there is no heat ( Dad & I did a set of tracks on an excavator that did a bit of work in salt water 10 years later give or take we were able to undo the track bolts )
Stockholm tar is old school pushed aside by the new products.It is brilliant to protect threads if there is not a lot of heat ie not for use on engines. But good for body & chassis bolts.
It is what I will be using all over the cab 12 when it starts to go together Sooner than later I hope