Sheaffer Prototype or Shop Model Fountain Pen

SH4721  Sheaffer Prototype or Shop Model Fountain Pen. No markings at all. Long steel nib with feed measures 2 1/8 inches. Where a cartridge would normally fit into the point end, there is a small black plug, permanently fixed. Black cap with integral ribbed one inch band, there is no inner cap, small cap opening where black plug at top of nib unit fits into. Bright steel angular clip looks like the plan could have been to turn it into a very modern looking reminder clip, however the clip is fixed. Measures 5.5 inches long from point end to top of cap. Quite unusual! (1 avail)