She started out as a 1918 BSA hence the '18 dated barrel date & for some reason got a replacement receiver in 1939, the receiver is the only Lithgow part on the rifle, all other parts seem to be the original BSA parts from 1918, would appear the receiver is a purpose built replacement receiver as Lithgow were still producing the MkIII in 1939.
Ian Skennerton has inspected this rifle personaly & thought it was interesting enough to include a small write up in his latest book The Lee-Enfield page 566 iirc.