I picture it as a legal pad, more or less. If I really think about it, I imagine a "legal pad" as having that very specific paper (lined, with that nice margin), whereas a "tablet" could perhaps be any type of paper bound together in that same way.

I'm not entirely sure where I got these impressions from over the years, though I certainly used to use a lot of legal pads. I still really like stumbling across a nice one in the wild, even if I usually just get them from Amazon nowadays. (Aside: Is it just me, or are legal pads not as good these days as they used to be?)

Anyway, from this bit on Wikipedia about legal pads, it seems like that is one origin story for using "tablet" in this context: https://en.wikipedia.org/wiki/Notebook#Legal_pad

Notably, from the last sentence of that story:

> ...he glued together a stack of halved sheets of paper, supported by a sheet of cardboard, creating what he called the "Silver City Writing Tablet".