Had to get some of these recently, good god you should have seen the wholesalers reactions!I've used these when I've had to fit deep accessories, provides an edge to plaster to when the box has to be fitted deeper.
Apparently you could have got them from electrical2go in 1-3 days.Finally managed to get them from our local independent wholesaler but still took a long time.
https://www.google.co.uk/search?q=steel+extension+box&tbm=shopI didn't think they were that uncommon??
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