This is quite definitely oak; apart from anything else, the mahogany Junior Monarch (page 27 in HMG) is a totally different design.
However this is also not the same as the normal UK Junior Monarch, and I suspect is is not a Junior Monarch at all, but one of the several European variants of the 'New Victor/ Victor Monarch model.
I am attaching (I hope) a scan of an old photocopy of the relevant page of the 1907 German catalogue. The dimensions given for the horn do not seem right for the horn shown, but may relate to a different (flower) horn, probably of a type not used in the UK. The equivalent model in 1906 had overall 'carving' on the case sides (several of that type have turned up), and was called 'Victor A' but G & T decided that the Victor name should only be used in the UK, so the German versions were from then on called 'Lord'.
The deciding factor is the tone-arm; the Junior Monarch has a full size tone arm, like a Monarch or Senior Monarch, but the Victor/ Lord has a smaller arm, 8 inches rather than 10 ½ inches (20/27 cm. approx)