I’m not OP, but I love his blog too! It’s like a collection of wisdom pearls.

Is it usual to type out what typewriter you’re using in every header?

It’s quite usual yes when you have a few machines at home. It helps remember the day you wrote that piece, because with typewriters the act of writing is meaningful by itself. I don’t do that, but I have only two machines, with two very discernable fonts (an elite and a pica).