I definetively agree on the 'Vovo' series being the most challenging.
Twelve different watermarks/varieties - plus unwatermarked!!
Scott's poor images of these watermarks does not necesarrily help the collectors very much, so I had to make my own overview of these 'Vovo -watermarks', for quick reference. Wmk 218 "in echelon" is not included in Scott. I figured this might be useful for other collectors to----? Print it out and use it with your 'watermark' station. Here it is: Let me know if you like a pdf (free off course)
(There is additionally a large 'Coat of arms' Sheet watermark (Wmk 236) used on some high values (1934 - 2$, 5$, 10$) but I did not bother to include it on the overview. These are better identified by paper type anyway).
So - what to be on the look out for???

If you find Scott no 259 (80R green Agriculture - normally unwmk) with wmk.100 - then you have hit gold

Same goes for the 'Aviation' 50R in red brown with Wmk. 100. None of these are included in Scott - so "completionists" out there might consider to update their album pages....
Here's my custom page for Vovo definitives, Casa Da Moeda.
The two boxes for the superscarce rarities will likely remain empty - hence I've included the images to remind me what to keep looking for....
