Is this an official tartan?

[figheadair]

I think it is missing the double black stripes, so there is only one black stripe in each sett.

Assuming you mean that there is a single pair of tramlines on each blue rather than the alternating 2/4 per the standard 42nd setting then whay you have is Campbell of Argyll per the Smith brothers' 1850 work.

There are said to be a number of Wilsons specimens of 42nd setting with a 2/2 arrangement i.e. a shortened 42nd count and it's possible that that's how this Lamont/Campbell started out although I have to say, I've never actually seen one of either.

It's possible that at some earlier point, assuming that it existed pre-1850, that your sett might have been called Lamont and/or Forbes but at this stage I'll stick to the Smiths' Campbell of Argyll.