The stamped metal sided trucks are correct for a 312 - the sintered/cast trucks came out with the knuckle couplers well after the 312 was out of production. It’s a definite improvement in looks adding the cast trucks to the tender (and adding a diecast trailing truck if the 312 has the bare wheel trailing truck).
MTN, you are correct about the stamped trucks. And my, are they ugly!
As for the diecast trailing truck, I ordered one of those online, right after I bought the 312, and it arrived early in the week. Now I'm waiting for the correct shoulder screw from The Train Tender, so I can mount it. (That same trailing truck also makes a big improvement in the looks of a 302, btw).
Meanwhile, I've been repainting the tender. And last night I installed a NOS bell on the 312's boiler front. (It was missing its original bell). Once I get the tender lettered, the only major operations left will be to install the LED headlight and fabricate a new drawbar.