So I took the recently calibrated Minolta LS100 illuminance spot meter from work and set up a 18% grey chart in daylight. (An old one at the back of an old Kodak data book.)
I adjusted the screen shades until I go got a reading of 1024 cd/m2 on the Minolta
This is supposed to be the same as an EI of 13.
My usual meter reads 12-2/3
The Pentax Spotmeter III reads 14
The older Pentax Spotmeter V reads 11-2/3
There is a parts diagram booklet for the Pentax 1/21 spot meter on learncamera repair.com
Richard’s website is http://www.lg4mat.net/ which has all of his contact info.