My 504 serial number is also in the 30xx range. It will shoot 3/8" groups (and a few better) at 50 yards with RWS 50.
I got mine at a salvage company, after they bought out a gunshop's inventory. I spied it in the rack from across the counter and noticed the beautiful woodgrain.
There are pictures and description of my accurizing process on RimfireCentral.com "Tips and Tricks Forum".