Okay, so I finally got the trials in hand, and hand a chance to image them. I think I did a horrible job. Anyway, the two yellow looking ones look a lot more like OLD brass than new brass, and the other one looks like old copper. Front/back/Front of 1971 50 baht copper trial obverse: Front and back of brass 1971 50 baht trial obverse: Obverse/Reverse of brass 1963 20 baht trial:
