From what I can tell it is a thermal detector rather than a thermal camera. I suspect it is aligned with the night vision image intensifier and displays a warning if a thermal source is present within the field of view. A kind of early warning system to alert the tank commander to a hidden threat that the night vision equipment cannot easily see. There is no thermal image as such produced, just a warning signal to the night vision equipment.
The detector is likely an uncooled thermopile or similar, as commonly used in modern IR thermometers. Not known for their fast response time but more than adequate in this application. It is basically a thermal 'spot meter'.
I used to own a couple of P4428 cameras but rehomed them to a museum/display project. The P4428 is quite old and could be considered collectible so prices have been rising. It is cheaper to purchase an EEV ARGUS 1 if you want a Pevicon camera. The early ISG Talisman camera also contains the Pevicon tube. We have an expert in Pevicon technology on the forum
Take a look at the Fire-Tics web site if you have not already done so.
http://www.fire-tics.co.uk/There is a forum there where you can ask questions and obtain help
There is a teardown of the P4428 and it's correct alignment on that forum as well.
Fraser