You could try the local history groups?

http://www.clhf.org.uk

Thanks Patrick, emailed them

I got a response from the Lake District National Park, saying:

there was a firing range in that area and was in use before and during WWII, the panels are likely part of the targets used by the home guard.

I do find that unlikely, as if you're going to shoot the heck out of something, you'd expect a bullet mark or two and there are none. Maybe Dad's Army were awful shots? Equally, why would you lug something so well decorated and heavy up a hill? There must be a better explanation.

https://cimg9.ibsrv.net/gimg/www.fod...52559dda47.gif
This is what I think about that suggestion.

I stand corrected. I've had a reply from The Lake District National Park people who say:

Hello Eric, I've heard back from Eleanor who has confirmed that the panels are targets. The site was protected as a Scheduled Monument recently, following the research that was undertaken and published by the Lake District Archaeology Volunteer Network. Please take a look at the attachment to find out more about the area. Let me know if you have any questions. Many thanks, Georgia

Great Grandad's Army.pdf

The pdf seems to put all doubts to rest, though I still believe they were awful shots.

Well I never. Thanks you for the delving. I am passing the PDF on to relatives who swill be interested in this history of the lakes.

And talking about Dad`s army New Stamps.