I have a working and a corroded analog 1/21 Honeywell Pentax Spotmeter.

The Big Button cell was a 1.3 V 640 mercury cell. There is now a 640A or A640 alkaline replacement which is 1.5 V. Some people say that screws up the calibration.

Someday I am going to do some 'spot checks', for lack of a better term, with a power supply at 1.3 and 1.5 & see what it really does.

I have a lab photometer with an 8 degree sensor that matches CIE curve within 2% & I've already seen significant differences between it and one of the
Pentax'es, with a 1.5 V alkaline battery...I do NOT have any fancy light sources, so I'm going to just change my distance from an illuminated 18% grey card (because it sounds like a good thing to measure off).

I note the spectral response in the manual for the Pentax Spotmeter is kind of lumpy compared to the CIE curve, so that probably contributes to differences a little too.

Some people have played around with reducing the output from a 1.5 V 640A to make it look closer to a 1.3 V battery. I don't know how that works out.

I don't know when I'm going to get to do the meter comparisons. It's at the top of my list, but unfortunately so is everything else, so nothing gets done!