Hi can anyone help me to identify what pin attachment is on my mast base? (See pic)
I am looking for a suitable mast extension and tried one made for a Euro pin but this one didn't fit.

AFAIK its euro pin but at some stage they made the inserty bit just ridiculously long and tapered, and consequently it won't fit some newer bases.
If the groove is at the right height, simply cutting some off the top of the pin may be all you need to make it fit all modern extensions.
It is an Euro pin, old North Sails I think. As it is quite old, check (or even better) replace the UJ before using it on the water.
Same as above, save yourself the trouble: don't use it for sailing! Replace tendons every season and the base every few seasons depending on conditions. Almost guaranteed this one will snap on first or second use if it's older than a few years.
PS. I learnt it the hard way ;)
Oh yeah and the washer is upside down too. The tab protrusion thing engages in the mast track to stop the washer spinning and rubbing on the board.
I have three of these older style bases. All of them more than 10 years old. In my opinion the old boge joints are better than the new tendon things. There is no need to replace them every year, I find they last at least 5 years. Just inspect them before sailing. Unfortunately, my boges are M8 at one end and M10 at the other. Australian stores don't stock them, but you can buy them from Europe easily and cheaply enough.
It's the little weak link that gives you grief. Buy a new Chinook base and it's one less thing you have to worry about.
If you have ever tried to reconnect the pin to a mast-base, while in waves/chop... this type of pin is lifesaver. Dont cut it off if it works in your gear.
I have both types, if I don't use the matching base, the extension goes on and clips but it's not seated properly.
Oh yeah and the washer is upside down too. The tab protrusion thing engages in the mast track to stop the washer spinning and rubbing on the board.
... or sliding so far back that the bolt can lift out of the track.
Thanks all for the great advice and insight. I've found a matching base and extension so hopefully problem solved.