I'm not so sure, John - I reckon this style could well be of the Harris period, or at least the Dobson period after that, ie. pre '75.
When it comes to the great big disc ones with the sea-urchiny shape inside, the clear and crizzle ones are considerably more scarce than the coloured ones. I can't honestly be sure, this is just my guts blethering. There really isn't a lot of "crizzle in clear" Mdina around.