Cast Iron cookware item. Purpose? Meaning of symbols, numbes on back?

I’ve had this piece of cast iron in my cookware collection for at least 30 years but have never truly known it’s purpose. I probably bought it at a flea market.

It obviously fits over the eye of a wood burning cook stove but I think it must have fitted inside an outer ring since it has a rounded tab on the end opposite the handle. The 2 side tabs have flat surfaces and one is smaller than the other.
The handle configuration also makes me think there was originally an outer ring.

On the back side the raised numbers “51” are easily readable but I don’t know what they refer to.
Then several inches from 51, there are two other raised symbols/letters/numbers I cannot understand. Orienting it in the same direction as when reading “51”, it looks like “31” with the 1 backwards....since I do not think they would make that mistake of a backwards 1, I’m wondering if it’s some company logo.

I haven’t found anything similar to this item in my looking at vintage cast iron cookware sites so I’m hoping CW can enlighten me on its use and what the info on the back means.

Thanks for looking!
Comments welcomed!