appears to have been machined flat and re-blued with no markings ...'

My 1918 Lithgowicon is like this too, a great rifle but with a civilian style re blueing over a not perfectly removed serial number, sounds like you've got a nice rifle, like mine. Mine came through Jovino NY NY originally, their mark is on the right rear of the receiver in tiny letters just above the wood line, about 1/16th tall, maybe yours is too?
Imine also has the correct cocking knob for 1918 and is lithgow, I'll take a look at the bolt head later ...