Like title says. Don't know if 20/12ga stock interchange? Youth would be ok. PM me with what you have and price. Thks!!
If the Express is like a traditional 870, I beleve the 12 gauge & 20 gauge stocks are different. The 12 gauge receiver has a slightly bigger 'socket' into which the front of the stock fits, with the stock sized to match, while the 20 gauge is slightly smaller (same size as 740/760 rifle stocks).
As always IIRC, FWIW, IMHO, YMMV, etc., etc.