From Royal Mail's Post and Go News webpage today (updated with latest information):
Royal Navy Museum Machines
From 19 October, printings at the four Royal Navy sites will be updated
to replace the current museum abbreviations. The Poppy stamp will be reintroduced from 19 October to 15 November, and the Machin will replace the Poppy on 16 November (But see below). The Union Flag stamp will remain on sale.
New printings will be as
The NMRN will change to Royal Navy + logo (Machin only)
The RMM will change to Royal Marines + logo (Machin only)
The FAAM will change to Fleet Air Arm + logo (Machin only)
The RNSM will change to RN Submarine + logo (Machin only)
The Union Flag and Poppy stamps
will just have the abbreviations replaced.
A location-specific logo graphic will be added to the Machin stamp design: the first day of use of this at the Royal Navy Museum will be 16 November, and at the other three 18th November (see below).
Royal Navy and Royal Marines machines only will carry a ‘Trafalgar Day’ inscription on the Union Flag design from 19 October until 15 November.
*Update 19 October
Correction provided by Roy in the comments:
Only the Royal Navy Museum is open on 16th November. The other three
museums are on winter opening times (closed Mon & Tues) and
therefore the first day for Machin will be Wed 18th Nov.
Machine A001 introduced the Lion design from the Heraldic Beasts issue
on 16 September. The Lion design will be temporarily replaced by the
Poppy stamp from 19 October until 15 November. The Lion will be
re-introduced along with the Winter Fur and Feathers stamps on 16
The change to the Royal Navy Museum machines will be welcomed if only because it avoids having to explain what the various abbreviations mean. Watch out for switched reels so that the Union Flag stamps have the logo!