That's a nice score if you indeed found a Lodge #20!
Those big old skillets are becoming harder to find.
The lack of a brand isn't a guaranteed clue since there are other early foundries that made unmarked pieces.
To narrow it down, you'll want to look for other identifiers like the handle style, how the size number is marked, and any other unique casting marks.
If you can post some clear pictures showing those details, members here can help you figure out what you have!
As for the value of a Lodge #20, a vintage piece in good condition could go for a few hundred dollars, and maybe even more depending on the specific series and the buyer, but it's hard to say without seeing it.