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".
“God save us from people who mean well.”
― Vikram Seth