Yes. A battery. Well, two, but one is a spare I am unlikely to need for a while.
I was given a Minolta SRT-303b 35mm SLR by Our Kid a year or so back. It's not taken a single exposure during its residence here. See, it's as heavy as a brick... Well, actually, that's only part of the reason. In fact, I didn't have a decent usable lightmeter at the time, and the camera needed an old mercury battery. Game over.
Ah, not so! Renewed searching recently uncovered a place where one can get, for an only slightly uncomfortable pile of cash, a suitable replacement. The two tiny batteries winged their way across the Atlantic, and I was lucky enough not to be hit with an absurd amount of duty this time.
Duly fitted, and the matchneedle swings across beautifully. I need to stick a film in and give it a whirl.