I have a number of arm daggers some of which are late 19c / early 20c. (but no real 'status' quality).
Now that the 'plastic' issue is sorted..... I am not certain as to age....had it been the 'aged' plastic I originally thought... then WW2 seemed reasonable ( I have seen a number of WW2 artefacts with plastic parts that have the similarly aged look and colouration.)
If the blade is from a machete ( AFAIK a number of Masai Seme have Victorian/ Edwardian machete blades ) it could be dated early 20c or even earlier ( the bone is quite old and degraded.) Also bearing in mind that it was 'hand filed' to shape.
I am not certain...but the 'style' of the stamped '4' seems 'old fashioned' but cannot find any other examples (of this type set, to prove/disprove this)
I do feel that the sheath, although it fits the blade perfectly, is a later replacement, the arm loop seems older....so perhaps reused on a newer sheath.
One thing I am not certain about is whether the lower end of the sheath is strapped / tied to the arm (in conjunction with the 'loop' ) to stop the knife/ sheath moving around, similar to a 'diver's knife'......but attached to the arm.
I too like these knives.......I think Lew does aswell