1931 Scout John Brown Motolamp – original shell or reproduction?

I recently found this 1931-type John Brown Motolamp shell at a small auction in Sweden. The auction house did not know what it was.

It had been repainted blue and later black. I have carefully removed those two later coats and reached what appears to be an older/original black finish underneath.

As far as I can determine, the shell and mounting arrangement are correct for 1931. The original-looking mounting nut takes an 11/16” wrench, and the internal brackets, rivets, screws and cable grommet all appear to have considerable age.

The shell has no MOTOLAMP stamping. I have spoken to a long-time Indian enthusiast who has worked with Indians for around 40 years, and he believes that the 1931 shells were not stamped. I have also noticed that “exact reproduction” 1931 Motolamps appear to have unstamped shells, which would seem to support this.

My main question is therefore not whether this is the correct type for 1931, but whether the shell itself is an original 1931 John Brown shell or an old reproduction.

Does anyone know when accurate reproductions of the 1931 Motolamp shell first became available? Were reproductions already being made in the 1950s–70s, for example?

And can anyone confirm from a known-original 1931 lamp whether the shell itself should be unstamped?

I would be very interested to hear from anyone who has handled both original and reproduction 1931 shells and knows of any construction details that distinguish them. 

Best regards Magnus