Please note the max pole length of 2320 mm (from face of mast) INCLUDES any sheet ring abutting the outer end of the pole. Recently confirmed by RYA Technical.
Ian Mackenzie has also reminded those who have fitted pole launchers to check that this has not resulted in over length poles.

So.... How is it measured with a musketeer style system? Is it 
just the mast ring + pole + sheet ring?

Just in passing, the length mentioned in the Summer mag is 2420

This must be a typo in the magazine.  Link to class rules below
 
See C.9.2: The relevant length measurement is 2320mm.
 
Whilst not a Merlin Class measurer, I would expect a measurer to determine a point which they think is the outer end of the pole.
 
I would then expect them to set the pole height so that is projects at 90 degrees to the mast with the pole swung forwards towards the jib luff.  I would then measure back to the mast.  The distance measured must not exceed 2320mm...
 
Clarification of the method adopted would clearly be helpful 
 
http://www.rya.org.uk/SiteCollectionDocuments/technical/Web%20Documents/Class%20Rules/Merlin%20Rocket%20Class%20Rules%2004.03.16.pdf

Adding up constituent parts I'm 10mm of max length. But in reality it's short as the pole sits next to the mast. Is that the price you pay for the musketeer system? Or can I make longer poles so when set it hits maximum from mast? 

Hi Matt.
 
The musketeer system is a bit of a grey area as the projected length can vary quite considerably depending on how you measure it. As a measurer I would take the maximum possible projection as the measured value, though I accept in practice the pole will almost never sit at that point.
 
Therefore if you're 10mm in at maximum projection you're ok.
 
The rules were not written with a variable projection in mind and we are looking at this. There is a school of thought that the poles are too long anyway, so I shouldn't lose too much sleep :)

 The link above isn't the latest rules. 
Google doesn't bring up the latest version and the link on the website doesn't work either.
 
It would be useful if they could be printed in the yearbook, I know it's not a controlled copy but the most up to date version could be printed with a web address to remind people to check to see if there is a more up to date version....
 
Q) What is the latest version, is it the one dated March 2010????

This looks like the current rules to me:

http://www.rya.org.uk/SiteCollectionDocuments/technical/Web%20Documents/Class%20Rules/MerlinRocket_CR_010917.pdf

Dated 2017. 

I have updated the link from our website to point at the most up to date rules held by the RYA which is the 2017 version.
 
Merlin Rocket Home page - About - Rules then click on Rules which is highlighted in red.