Greeting from Finnish Lapland!

I have just found this document - The gamma counting handbook. It has many references to Wallac, but I think from the 80's or later. It's a long read so one for the weekend I think. It may shed some light on what is in your Wallac. It does mention a 'well' type detector. Not something I have heard of. It may be similar to an ion chamber, but one diagram shows it with a PMT (photomultiplier tube). It does look interesting even if it has no bearing on your Wallac.

https://ia802907.us.archive.org/3/items/manual_PerkinElmer_Wallac_1470_Wizard_1480_Wizard_3_Gamma_Counting_Handbook/PerkinElmer_Wallac_1470_Wizard_1480_Wizard_3_Gamma_Counting_Handbook.pdf

Very nice! A thing of beauty and a joy forever! I really need to get my collection documented. Maybe we should have a forum just for vintage counters and and related equipment? 

I'm very curious about the detector. Six wires? It may be the type that is mentioned in that document I posted. Looking at the two trim pots for calibration I do suspect that it is designed for high levels of radiation. If you ever see that meter move, run!