There were various imperfections, particularly in the typography process, regarding alignment of the words and the placement of the ornament*.  The differences in alignment are very slight, but measurable.  These imperfections, unique to each position,  were reproduced on each sheet printed making it possible to identify in which position a stamp was when it was overprinted.

* An ornament was placed between the words "POCZTA" and "POLSKA".  Form III had no Ornament,
Forms IV A and IV B had a Decorative Ornament. 
Forms I, II A, II B, II C, II D and II E had a Rhombus Ornament.
 
The Number of stamps overprinted per form.

Form	# of stamps per sheet
I, II A, II B, II C, III, IV A	100
II D, II E,	50
IV B	25 (Form VI A, partially used)

 
Type of Overprinting and the Printer

Form	Printing type / Printer
I, II	Typography (printed by Kozianski)
III, IV	Lithography (printed by Zielinski)

 
Upside Down and Double Overprints
Some sheets of stamps were accidentally overprinted twice and other sheets had the overprint mistakenly applied upside down.  These stamps are quite rare.

6, 40, 50, 60 and 80 Halerzy Regular Postage  15 Halerzy Postage Due	Upside Down Overprints exist
40 and 80 Halerzy Regular Postage  50 Halerzy on 42 Heller Postage Due	Double Overprints exist

 
In addition there were imperfections of the letters themselves.  The 1 korona example below is known as Form I, position 30.  The "A" in POLSKA is damaged.  Stamps printed in position 30 on Form I will have this flaw.  Other less obvious flaws exist on other letters for other printing positions.  The combinations of the alignment and flaws in the characters are used to help authenticate these stamps.   These characteristics will be duplicated on good forgeries
Broken "A"- Form I, pos. 30.
 

Value	# Produced	Form
3 Halerzy	2,350	I
5 Halerzy	2,050	I
6 Halerzy	28,800	I and II E
10 Halerzy	2,250	I
12 Halerzy	15,640	I
15 Halerzy	62,750	I
20 Halerzy	5,340	I
25 Halerzy	1,100	I and II B
30 Halerzy	3,740	II B
40 Halerzy	22,390	I
50 Halerzy	58,230	I
60 Halerzy	67,020	I
80 Halerzy	74,190	I
90 Halerzy	1,300	I
1 Korona	50,860	I and II B
2 Korony	90,275	IV A and IV B
3 Korony	8,005	IV A
4 Korony	5,508	IV B
10 Korony	440	IV B

25 Halerzy on 80 Heller

100,000

III

 

KRAKOW PORTO -Postage Due
 

Value	# Produced	Form
5 Halerzy	34,000	II A
10 Halerzy	150	II A
15 Halerzy	76,000	II D
20 Halerzy	450	II A
25 Halerzy	8,760	II A
30 Halerzy	270	II A
40 Halerzy	1,000	II A
1 Korona	80	IV C
5 Korony	80	IV C
10 Korony	15	I  -Black overprint
10 Korony	40	IV C -Red Overprint
15 Halerzy on 36 Heller	600	II B and II C
50 Halerzy on 42 Heller	6020	I

 
Krakow Newspaper stamps on Austrian Newspaper stamps

VALUE	# Produced	Form
2 Halerzy	23,150	II C
4 Halerzy	56,350	II C
6 Halerzy	71,195	II C
10 Halerzy	11,350	II C
30 Halerzy	34,950	II C

Production numbers denoted in orange or red signify rarer items.
 
Good page with info on Krakow Overprints 1919
http://home.att.net/~mikemelni3a/