Actually we know exactly when those bisects were 'crafted', and that is virtually when the stamps were issued.
Apart from the current Fischer Polish catalogue there are five main references I know of for the Warsaw Municipal Post.
1) 'Polskie Znaki Pocztowe', Bialystock, 1935. pp.196-225
This was reprinted virtually verbatum in
2) 'Polskie Znaki Pocztowe' Vol. 4, Warsaw 1966. pp. 1105-1130
Based on the collection & research of Stanislaw Rembielinski who was the first to seriously study these issues. He had access to the Post's archives in Warsaw as well as the printers of the stamps.
Unfortunately the Warsaw archives were destroyed in the Warsaw uprising, however, unbeknownst to Rembielinsk, Karol Jenke, the Organisor & Director of the Warsaw Municipal Post, had deposited much archival material in the Postal Museum in Wroclaw & much of this is still available which leads us to the 3rd reference by the philatelist & keeper of the Postal Museum, Alexander Sniezko.
3) 'Poczta Mieiska Miasta St. Warszawy 1915-1918', Alexander Sniezko, Warsaw, 1965.
This booklet of 143 pages expands on the information previously published with photos of the buildings & people involved, stamp printing figures, postbox locations, etc.
Finally, I have
4) 'Stadpost der Hauptstadt Warschau 1915-1918', Stefan Petriuk, Langballig, 1980.
Stefan is a contemporary collector of Polish material, he has posted some of his collections online & they are all mouth-watering in their scope. His booklet of 90 pages has much of the same material previously published but includes other photos & illustrations of covers & cards relevant to the Post & is in German as opposed to the first three which are in Polish (why didn't I pay more attention in German class 50 years ago?)
As previously mentioned
5) 'Poland 1918 Locals' J. Barefoot Ltd., now in it's 3rd edition.
This is a must-have for English-speaking collectors & is a comprehensive catalogue of the stamps with much info on the forgeries, reprints etc. & the latest edition is in colour, nice.
A couple of things come out of the info available here which reflect on Ben Nieborg's pdf file of the Post.
The first is that virtually all of the colour varieties, imperfs, inverts, perf varieties, overprints in different colours, etc. were no more than machinations of the printers at the time. The back door of the printers led straight to the dealers of the day. There are proofs of some of the stamps, these are listed in Fischer, but in such small quantities that we rarely see them.
The second thing to note is that most cancelled bisects are philatelic in nature. The Municipal Post never authorised the bisecting of stamps. Rembielinski undertook a study of the bisects & found that maybe on the 5/12/1915 & 3/3/1915 non-philatelic bisects were used.
So as to ease the burden on the Post's employees in regard to philatelic mail, the Post loaned their cancellors to stamp dealers, you can see where this went, the dealers had a field day.
Here's my copy of the unnisued first issue
Looks good doesn't it - well it isn't. The genuine article was printed in black, under 20x magnification you can see that my reprint is brown-black. You see these on eBay
, buyer beware...
I have a basic collection of stamps & covers of this issue, nothing out of the ordinary, except for this one item which is a proof of one of the unnissued set of four which were overprinted in 1918 for use in newly independent Poland