Pirate, thanks!
And I do believe what you say is true - many collectors from MPF seem to have been much more into contemporary posters whereas, generally, but not without exception, many of the people from NSFGE prefer the vintage material. Many collect both. However, personally it is VINTAGE all the way! The contemporary slant of MPF is one reason why I virtually never visited or posted on that site.
To answer your question on how many of these posters exist - certainly not nearly as many as with US paper on the same titles (these are Australian daybills which generally had much more limited print runs owing to a smaller population and distribution). There seem to be several Spider daybills that have shown up recently. The Mummy on the other hand virtually never turns up. This may just simply be a case of collectors holding on to them. Another explanation is that Australia has strict rules with respect to censorship (even to the present) and so horror posters and posters that depict lurid or overtly graphic images (although there are exceptions) seem to be somewhat scarcer.
The Mummy and the Spider were both acquired through good old eBay (I use the 'good' loosely). I had searched high and low for the Mummy and even tried to bribe Ari into giving up his (with little to no success
) before one finally turned up for sale. Now it will be in my collection likely forever, framed happily on the wall!
To be clear, these posters are no where near the scarcity level of the Black Cat poster you referenced and many Universal horror titles from the same period.
