Radiator cap

[updraught]

Yep, just did a search:

"To replace the thermometer, you have to remove the metal plug at the bottom of the Motometer probe (the portion that extends down into the radiator neck)--it's just crimped in place, and then "dig out" the grout that surrounds and secures the glass thermometer--use old dental scrapers for this, they work just fine.

http://forums.aaca.org/topic/91029-boyce-motometers/