This is a most interesting topic, and honestly I do not think ever really brought up in all the years here, that of the Mau Mau rebellion and certainly not these incredibly fascinating ersatz guns.
For me most of the exposure to the curious term, like many people in my age group (really old) came from the novelty song of early 60's "Oom Papa ooo Mau Mau' and the term 'mau mau' used rather collectively in various cases. Probably from the British colonial slang term for the Kenyan fighters.
There does not appear to be any viable etymology for the rather derisively applied term, which was of course not used by the tribal groups involved in these uprisings leading up to the Mau Mau Rebellion (1952-60). This was but one of the many and constant struggles in colonial Africa, and while these were it seems constantly in the news in the 60s, it was hard to really understand what these were about for the average lay person, nor especially where these places were.
From what I read in the links Tim kindly added here, the mostly Kikuyu tribes among other tribal groups involved used these kinds of improvised and even imitation guns as symbols of rank and status, when actual serviceable weapons were unavailable. The suggestion that this was the case with many of the 'Mau Mau' tribal leaders would seem to elevate these rough, crudely assembled weapons to rather unique collectible status.
Indeed, there seems to be numbers of these types of tribal guns in the collections at National Army Museum in London.
It seems there are examples which fall into the popular term 'zip gun' type of crudely assembled guns innovatively created as mentioned by Tim.
Interestingly one of the key creators of many of these 'Mau Mau' guns was a self styled gunsmith who used the name "General Doctor Russia", rather a bizarre but catchy title, located in remote areas near Mt. Kenya foothills.
Examples of military type rifles with bolt action even with dummy magazines and pistols with open breech etc. are found in these groups of imitation and improvised guns.
As noted, this example posted by Jack seems to have been a sporting shotgun, which seems unusual as mostly double barrel versions were the norm. Also as such sporting guns were made by notable gun makers with ornate and signed locks, this crude assembly is entirely out of character.
The markings stamped into the stock are interesting, and it seems that there may have been some copying of 'official' issue type markings and numbers used in imitation of military guns with these improvisations.
Most interesting topic and example Jack! Thank you for posting this.
Rob and Tim thank you guys for your valuable input and Tim for the links.
I really enjoyed learning more on this topic, which really has my interest piqued now as an area I know even less about than guns!!! and that is most notable
All best regards
Jim