Repros are commonly found with either the top rib inscription or, like your new-to-you 20, on the left barrel. I have seen more having the markings on the left barrel than on the top rib and I do not recall ever seeing a 28-ga. with the markings on the rib. Does anyone own a 28 with the rib inscription?
Here's a previous thread on the (or lack of) "Made in Japan" stampings.
http://parkerguns.org/forums/showthr...ighlight=Japan