Here beginneth the second lesson, and the topic this week is provisional postage dues in SHS/Yugoslavia.
Yugoslav post offices frequently ran out of Postage Dues. This is NOT because Slavs were especially prone to understamping correspondence; nor was it that Postage Dues were produced only in small numbers. The problem was that so many of them were used for payment of a variety of other Post Office fees such as parcel storage, Value Declared fees, poundage (dinarage?) on Postal Orders, and fees for postal receipts. This is an example of a usage on the back of a Postal Order:

So periodically, whether on a local or a national level, they ran low on Postage Dues and solved the problem temporarily by using overprints to create provisional postage dues. Three sets of these are well known to everyone, being listed in Gibbons and other standard catalogues:
On Bosnian issues

On Slovenian issues

On SHS national issues

What is far more controversial is the handstamping of Porto or P by local postmasters:


These are not listed by Gibbons and Gibbocentric collectors persistently poo-poo these issues and tell everyone who will listen that we should have nothing to do with them because they are all fantasies aimed at milking philatelists. That some of them were sold on covers to philatelists, and quite possibly to dealers as well, is undeniably true. This one for example:

But exactly the same would be true of the bulk of modern FDCs. In fact I have no doubt that many of the provisionals did genuine postal service. Let's use this as an example:

Before the end of World War 1 an Austrian Government Office in Marburg an der Drau (soon to become Maribor in Slovenia) distributes Portofreie envelopes to carry replies to their correspondence. Shortly after the end of the war one of the recipients of this largesse attempts to use it. He initials it lightly in the bottom left corner and posts it. However, the newly independent SHS Post Office sees no reason why they should be deprived of income through documents issued by the erstwhile enemy, and slap 20v+20v worth of provisional handstamped Postage Dues on it. This is a perfectly reasonable and definitely non-philatelic usage.
Also the 'T' (=Taxe) handstamp was sometimes used on a stamp to create a postage due, rather than being applied to the cover to warn the receiving office of an underpayment:

Although again these were also used to create philatelic covers (13 vinar postage due would imply a postal rate of 6.5 Vinar which never existed; nor, come to that, did half Vinars!):

While on the subject of these, similar overprints were used on prewar Montenegrin stamps. I have grave doubt's as to whether these were ever genuinely used; but if anyone has seen them used on cover, please upload a scan:

Equally controversial is the use of captured Postage Dues in areas of Hungary which were occupied by Serbian forces at the end of WW1:

Again they are usually condemned as purely philatelic. In this case I suspect that their overprinting was more a political gesture to establish a claim to the area, than anything philatelic.
On the other hand (to show that I don't fall for everything!) I have no confidence at all in the genuineness of either of these - but again I would welcome correction:
Slovenian issues:

Slovenian provisional Porto issue additionally handstamped Bosna (not required since Slovenian issues were automatically valid for use in Bosnia):

And just to finish off - the great irony given the postage due shortage: postage dues overprinted Franco as provisional postals!
