What size electrical junction box fits a 1-inch diameter cable anyway?

You are right of course about knowing the answer, the comment was made as friendly advise. It is common for people new to a task to get too technical and ignore common sense or reasoning. Anyway whatever, you are on the way to the knowing part :-)

Thanks for that friendly advice.

The problem is compounded by the lack of details in the part number.

For example, I can get "buna" rubber O-240 o-rings for 50 cents each:

Or, I can pay $16 (on special!) for that o-ring at the pool supply store:

The problem is in knowledge.

For example, is the sixteen-dollar o-ring made out of the same buna rubber material as the fifty-cent o-ring?

I don't know. Do you? How can we tell?

Note: If we already know the material is the same, then it's not a technical problem; but if we don't know the material - one has to wonder whether you get what you pay for. That is, is a sixteen dollar o-ring the same material as a fifty-cent o-ring?

There's where it gets technical.

That $16 O-ring represents the cost of the supply chain.

If you want to educate yourself to reasonable level about O-rings & O- ring material apps,' go to McMaster Carr catalog and search on O-rings.

If you REALLY want to learn about O-rings

Considering the duty go for the cheaper part, many of the pool shops will gouge your eyes out on parts that are quite often the same as those available elsewhere cheaply.