Thanks for posting these Rod!
I don't think there is a thread on this forum specifically dedicated to Malta's stamps - so if anyone has anything to add or would like to ask any questions, I'd be happy to help out if I can. :D
Here are couple of comments about Malta's revenue stamps for members who are unfamiliar with them.
The islands introduced revenue stamps in 1899, and for the first few years they were produced by overprinting postage stamps. Some overprints were produced locally at the Government Printing Office in Valletta while others were produced by De La Rue in London. They are easy to distinguish, as local overprints were mixed-case while London overprints were always uppercase:
From 1926 (not 1925 as stated in some sources), a system of key types was introduced, similar to the revenues of Britain and India. These stamps had a tablet at the bottom which could be appropriated with a specific inscription - in Malta's case there were issues for Applications, Contracts, Registers and Stocks & Shares, while an unappropriated set with blank tablets was issued for general purposes.
The images below show British, Maltese and Indian key types as well as a Maltese issue with different appropriations:
Starting from the late 1920s, Malta also issued revenue stamps with distinct designs for specific purposes, such as Workmen's Compensation, Passport, Airport Charge and Excise as illustrated below:
Taxpaid excise issues like the one at the bottom are still in use today for cigarettes, tobacco, alcohol, lubricating oils, etc.
Apart from that, there were dual-purpose stamps which were equally valid for both postal and fiscal use at various points between the 1880s/1890s up to around the 1990s. Fiscal cancels on such stamps (as well as revenues proper) were traditionally derided by many collectors and were not regarded as being worthy of a serious collection.
An almost-complete though unpriced catalogue of Malta's revenues (plus some cinderellas) is freely available at https://www.revrevd.com/malta.html