I've been defeated. It won't come out (;_;)
I gave up searching for the part and made my debut in hot water molting.
From a completely rough state with no precision whatsoever, I somehow managed to mold it until it fit into the side skirt...
And as you can see in the thumbnail photo, it's successfully restored?

Today, as a reaction to that, I even did panel lining. I'm using the same "whitish something" as with the Astray, but the color development is definitely different when the base coat is different. Last time with the Astray, I used black primer, this time it's gray primer.

Hmmmm! The parts-hiding monster has appeared! | インフラビビンバ's Gunpla works