@Mike Smart
Caveat - I'm no expert on Gibbs razors - but I owned a #15 adjustable model for a while - a very fine razor - that did need the blades adapted to suit modern ones - not such a great effort - you use a pair of scissors or a hole punch/snipper type thing. The base plate and top cap, from memory didn't look like that - looks more for standard blades to me. I'm sure the adjustable had lugs at either end of the base plate to secure the blade. You might ask
@Blademonkey - as I bought it from him. Yours - I.
Oh - Mike - I like the fact the vintage blades have 'mince' on the packaging!