Hi Dan
I agree it's a bit of an enigma.
As you say the support arm looks short and the underside is totally un Deckel like in both shape, end stop and machining quality.
I also noticed the quill feed shaft looks wrong with no graduation dial.
I just realised the support arm dovetail runs the full length of the support. There is no cutaway section to drop the head over the fixed accessory drive gear, it cannot slide onto the machine without hitting this gear wheel and will need modification.
So my original question is somewhat academic and I wonder what it's from, most odd.
Ciao Adrian.