Will a youth stock fit on any Lt 20?

Maybe?

Not trying to be a smartass, but over the years Remington has used several different 20 gauge receiver sizes.

You could have two of the exact same model, made a year apart, and the stocks won't interchange. The only way to tell is to either try it, or take some measurements.

What vintage and model are you referring to?  I think 1100, but want to confirm.

I have an 80's vintage 870LW Youth and an 1100 and 1187 stock will fit on it perfectly.   So I would think were I able to remove my 20 gauge youth 870 stock and slap an 1100 LT stock on it, than the reverse would be true.

I picked up a few gorgeous stocks when "green parts store" was still a thing and they clearanced some.

I have a 1100 LT 20 and a 11-87 20 and the stocks interchange.