Australia Post "Specimen" Stamp Folders
These were sold by AP from 1913, and the price for most sets for the next 50+ years was £1.
The state GPO philatelic Bureaus all allegedly had them on sale if you asked. In those days there was zero Philatelic mail order to speak of, or any marketing by them, indeed no 'Bulletin' in this era of any kind, so they were largely overlooked.
Each folder contained corner CTO copies of the values up to 5/- in general, and over that they were overprinted "Specimen". (There are exceptions, the 10/- 3rd Wmk perf OS corner CTO is one, but that was NOT the general rule.)
The stamp market worldwide would basically have no used 5/- Bridges in it, if it were not for the 1,000s of CTO copies that appeared in these packs.
The folders opened up, (mostly tri-fold as shown above) and there were lines of glassine strips to house the stamps in .. a little like today's stockbook pages.