1920-24
Ships in service
From 1920 to 1924 the ships serving the New Hebrides were "PACIFIQUE" for Messageries Maritimes and "MAKAMBO" for Burns Philp & Co. Ltd.
The Goyns/Klinger postmark PM6 can be found on many New Hebrides covers between 1 May 1924 and mid of January 1932.
On very few covers it can be found as a variety which is remarkably different: The difference can easily be seen with the naked eye:

Goyns/Klinger PM6 a
This is the standard type PM6 a

Goyns/Klinger PM6 b
This is the scarce type PM6 b

Goyns/Klinger PM6 b over PM6 a
This is PM6 b over PM6 a
These are the differences:
PM6 b has a comma after VILA and a wide gap to NELLES, PM6 a has a dot after VILA and a much smaller space. Especially PORT has narrower characters compared with PM6a. The upper writing is shorter and therefore the two dots at
left & right are higher in PM6b. The double ring with the inscription of PM6b is tighter and the postmark is a bit smaller. The divider between POSTES and PORT is a dash in PM6a
and a dot in PM6b.
Here are the covers and dates with PM6 b know to me (as far as I can see on the scans):

6 June 1924
The three 1924 French provisionals and three British provisionals on a cover without address.
Seven nice impressions of the scarce PM6 b. (Klinger coll.)

6 June 1924
Cover to Nantes, France. Must have had an extra paper on front with the address. No trace to
be seen of an address but a small ink dash near the extra cancel. The letter is
correctly backstamped NIMES, LOIRE INFERIEURE 28 - 7 24". All stamps cancelled with Goyns/Klinger type PM6 b.
(Klinger coll.)

7 June 1924
The three 1924 French provisionals on a registered
cover to Victoria. Type R2 registration label not used. Goyns/KlingerPCH type PM6 b. (Klinger coll.)

7 June 1924
The three 1924 French provisionals on a registered
cover to Victoria. Type R2 registration label not used. Goyns/Klinger type PM6 b. (Blinco coll.)

7 June 1924
(new 5/2018)
PM6b on a cover to Parkville, Victoria. Stamps on reverse. Seen on Ebay in 2011.

7 June 1924
(new 5/2018)
Reverse

7 June 1924
(new 1/2019)
The three 1924 French provisionals on a registered cover to Victoria. Type R2 registration label not used. Goyns/KlingerPCH type PM6 b.
Image thanks to Sandafayre - (colors altered a bit)

26 June 1924
Cover to Melbourne, the 75c stamp cancelled with the scarce PM6b. (Ruecker coll.)

26 June 1924
The GK PM6b postmark.

18 August 1924
(new 1/2019)
Registered cover to USA, even as heavy letter overpaid 10c. The 1F stamp ist cancelled with GK PM6b.
On reverse: Sydney REGISTERED 28 AU 24 - Brooklyn OCT 9 24 - Brooklyn Cooney Island OCT 9 24 - 10-10-1924 New York C.H.STA.REG. - 10-9-1924
New York REG.Y.DIV.
S.S. MAKAMBO arrived at Sydney on 28 AU 24.
The Cortlandt STamp Co. was situated at 337 Gates Ave, Brooklyn at that time. (Klinger coll.)
reverse

18 August 1924
Type 6b postmark

18 August 1924
Registered envelope to France, the French 1911/12 1F stamp cancelled with GK type PM6a on the same date. Registration cachet KG type NR3.
This must have been a 70g letter as the colonial letter rate was 20 c for a 20g letter and 10c per 10g and registration fee was 30c.
Backstamped AYDAT 7.10.24. (ex Millet coll.)

18 August 1924

19 August 1924
Two 30c stamps on a registered letter to France cancelled with Goyns/Klinger type PM6 b in red, date block inverted.
Registration handstamp Goyns/Klinger type NR3. Backstamped Valence 6-10 24 and postmaster
cachet "Condominium Postmaster, New Hebrides" (Holland type oc1). (Klinger coll.)
This 18 AOÛT 24 is an interesting date: The "18" on the cover to the U.S. above is rather clear and safe and at this date MAKAMBO arrived in Sydney.
BUT: besides this one there is another cover to France with PM6 from 18 AOÛT 24 which very clearly carries PM6a - date block not inverted.

18 August 1924
(new 5/2018)
This postmark is clearly PM6b - maybe they had difficulties with the date.

18 August 1924
(new 5/2018)
The same postmark turned by 180°

18 August 1924
(new 5/2018)
The other 18 August postamrk from the cover nearby which is PM6a.
News: this is the first time that I saw the very scarce postmark PM6 type b on a stamp and even with a so far unknown date: 14 JUIN 24.
Should you find a PM6 type b in your collection please send me a hi res scan or at least the date so we can see the usage.

PM6 subtype b - 14 JUIN 24
PM6 subtype b on a 1924 50c ovpt

PM6 subtypes a & b
PM6 subtypes a (at right) and b (at left)
These dates of PM6 type b usage are known to me so far:
6 JUIN 24 -- 7 JUIN 24 -- 14 JUIN 24 -- 26 JUIN 24 -- 18 AOÛT 24 (date block inverted) -- 19 AOÛT 24 (date block inverted).

PM6 subtype b - 14 JUIN 24
PM6 subtype b on a 1924 10c ovpt ("SeSi" coll.)

PM6 subtype b - 14 JUIN 24
PM6 subtype b on a 1924 30c ovpt - the "POR" tells us that it is PM6b (Klinger coll.)

PM6 subtype b - 14 JUIN 24
PM6 subtype b on a 1924 50c ovpt ("SeSi" coll.)

PM6 subtype b - 14 JUIN 24
(new 6/2020)

PM6 subtype b - 14 JUIN 24
PM6 subtype b on a 1924 1d ovpt ("SeSi" coll.)

PM6 subtype b - 14 JUIN 24
(new 3/2019)
PM6 subtype b on a 1924 1d ovpt (Jeff Blinco coll.)

PM6 subtype b - 14 JUIN 24
(new 6/2020)