Ok, so..
It wasn't a slashing. The tires are just old.
A second one went out recently.

Now i have a new task...

What is a good set of tires for a 1985 300D Turbo, that is ALL WEATHER?
I would greatly prefer to not be swapping tires per season.
I'm ok with a little loss of performance during summer, or whatever sacrifice it may be to just keep a set of tires year round.

I can also pump them up a little fuller in the summer, i assume.

Today, after my second repair, i brought them all down to 28 PSI.