At first glance it’s a Swan SF1 in unusually good condition. Picking it up, it seems a little too heavy – and that’s because it’s solid, machined to the appearance of an SF1 only externally.
This would have been a salesman’s example or, more likely, part of a shop display. I suppose it saved the labour of making an actual pen and, of course, the cost of the nib. It’s just a pity that the actual pens turn up in such good condition so very, very seldom!
These things appear from time to time but they’re by no means common. Most, I suppose, were thrown away long ago when the model ceased to be current.