Hi David,
My thoughts on your 5C continued... (Yes, it's 2 AM and I'm doomscrolling.)
1) Canada would have received the specifications for the Switchboard No.5 as part of the WS19 manufacturing package, as it was standard for wireless trucks.
2) This would be the original (Mk.1) switchboard with the plywood back panel.
3) I suspect the UK deleted the back panel to save resources at some point - it's not necessary when screwed to the (wooden) wall of a "Wireless House" body, so can safely be dropped. For ground use the users can simply ensure it's not plonked down on anything conductive - and at 12 volts it's not going to arc over.
4) The stores code on your switchboard should be "5A" rather than "SA", and if you enlarge the image, I think the "S" is an upside-down figure "5". Since the lettering was produced by stencil or pantograph engraving machine, I think this is due to the stencil (or guide plate) for the figure being put in upside down and not noticed.
5) Canada redesigned the Wireless Set No.9 into the Wireless Set No.52, using readily available valves (tubes) instead of the single-source STC ones. I suspect they also reworked the charging switchboard to be a more generic (Charger No.1/Charger No.2) instead of (Vehicle Dynamo/Charger) and (12v Output From/Interior Lighting From) instead of Charge Batt 1 and Charge Batt 2 as on/off switching. This allowed the use of two chore horse chargers for the WS52 and was considerably more flexible.
I'm sure I've seen a plastic (Black Bakelite) switchboard with engraved and wax/paint-filled lettering, but the two I have appear to be slate, and are extremely heavy. (It's like lifting a paving slab.)
Thanks for posting the (Canadian) circuit diagram, I can use that to understand the British switchboard - that appears to have been unchanged throughout its life.
Best regards,
Chris. (G8KGS)