Greetings from the future! The 1 degree analog Pentax Spotmeters are confusing because they all look the same and most are not labeled clearly with a distinguishing model number. Here is the cheat sheet:
Spotmeter I = 3 degree meter with right angle viewfinder: uses one 1.35V "640" mercury battery and one 22.5V "412" battery.
Spotmeter II = new design 1 degree meter with CdS light sensor: uses one 1.35V "640" mercury battery and one 9V dry cell battery. Dark brown case.
Spotmeter III = a Spotmeter II with backlight: uses one 1.35V "640" mercury battery and one 9V dry cell battery. Dark brown case.
Spotmeter IV = no meter sold with this model designation
Spotmeter V = a Spotmeter III with SBC light sensor: uses three LR44 alkaline or SR44 silver oxide batteries. Light beige case.
Note that the 1.35V "640" mercury battery is double the height of the mercury batteries usually used in camera equipment. Don't replace it with two modern batteries, use one battery and something (conductive) to take up the extra space. Zinc-air hearing aid batteries are good electrical replacements for the 1.35V mercury batteries but a physical adaptor may be needed in the battery compartment.
The Spotmeter II and III models have a dark brown case, CdS sensors, and two non-linear L (3-10) and H (10-18) EV scales in the viewfinder. Meters sold in the USA have "Honewell Pentax 1˚/21˚ Meter" on the top while those sold in the rest of the world have "Asahi Pentax Spotmeter" on top, neither have a model II or III designation. The easiest way to identify the Spotmeter III is the backlight button on the handle.
The Spotmeter V has a light beige case and was sold in all markets with the label "Asahi Pentax Spotmeter V" on the top, which makes it easy to identify. It has one linear 1-19 EV scale in the viewfinder.
Recommendation: The best and most useable analog Pentax meter is the Spotmeter V with the light beige case that uses three LR44 or SR44 batteries.