I'm with Larry on the spark plug. Doesn't look as new as the other 3. Check and see if it has a different p/n on the porcelain insulator. If the same #, inspect the threads closely and you will see the helicoil threads under the gunk. You can probably install a new helicoil with some Loctite and be good.
I'm with Rex on the check the heads for cracks too. Typical cracks are from valve seat to spark plug hole or between valve seats. They can be very obvious or teeny-tiny hairlines so inspect carefully. If they're cracked, scrap the heads. Check out "Need VW type3 DP 1600 heads" thread for pix.
Welcome to the world of old VW engines.