Meters appeared first as short tests in the US, New Zealand and Norway.  In most places it took a long time for the first commercially accepted units to be placed with customers.  This gallery attempts to show images of the first commercial meters in the first few countries that approved their use.  

Images from The International Postage Meter Stamp Catalog, 1st Edition 2005

ASC Moss Model 3 A6	New Zealand 8 February 1906 New Zealand started vending meter tests as early as 1904 but these were rather short lived.  It is perhaps a risky value judgment but this model seems to be the first commercial unit that was used for a significant time.
Pitney Bowes Model M B1	United States 10 December 1920 Tests are known in 1897, 1912 and 1913 in the US.  The first commercial meter was mailed from Pitney Bowes offices on 10 December 1920.  The earliest know use outside Pitney Bowes offices was 15 April 1921.
Moss A1	Egypt 27 February 1922 This Moss meter was installed in Egypt.  Each value had a different indicia.
Pitney Bowes A1	Great Britain 23 August 1922 Perhaps the next commercial installation was in Great Britain.
Pitney Bowes Model M A1	Canada 29 September 1923 The early indicia were hand cut and some of them had exaggerated curves ending the lines under Canada and above Postage. Production indicia looked like the one shown here.