Well, we speak about german proof stamps which are the following:

(always with crown) G= Gezogener Lauf (rifled) , S = Glatter Lauf (smooth), W = Wrgebohrung (choked), S with W inside= gezogene Wrgebohrungslufe (rifled ! chokes) the letter must be proofed with a solid projectile!!! I think, that's what you mean on the austrian guns. After regarding this, you never will find a smoothbore tube with a crown over G..... (Do you know one?)

Kind regards,
Wolfgang

http://www.hunting-heritage.com