Looks ok but I don't know if it's an RDM or SDM, You might need to ask the question. You'll get 2 mast for one, A 420 and a 400. You may just need a longer extension to make up for the 10cm you are short on the 420, It's an odd size....Must be a north thing?
As for the Euro pin Vs the US cup, Everyone will have their own opinion. I like the pin, Simple and it works. I've seen a few US cups get stuck with sand but people like them for their secure fitting.
Chinook makes quality gear and you can't go wrong, Billows on the other hand is a bit of an unknow. People have said on this forum that they have experienced problems with some of their masts. That being said I have a 100% carbon RDM of theirs and it's being performing fine for about 3 years with no problems, Even when I put in a Pryde sail....So it must be ok

Cheers.