scotts33 Exactly that, it was the first (in 1860), and the only one at the time so didn't need to be anything other than The Open. And yes, The Masters is and always will be The Masters, never The US Masters.
It's not pretentious, it's just a fact. It's not, and never has been, the British Open. Look at the flags, and the billboards around the course. I'll change my opinion when they print "British Open" on them.
And if you want to attend the 150th, at St Andrew's next year, the ballot registration is open now and likely to be your only chance of a ticket https://www.theopen.com/150th-ticket-ballot