I'm throwing a bone stock 69 340 in car car to get it running while I'm slowly working on my 470. I thought this engine was stone stock but the pistons say 266-99 on the bottom. Not wanting to tear this engine down other then changing oil pans I googled the numbers and found someone said these are old trw forged pistons. Is that what they are or misinformation?

All the old TRW or Speed Pro 340 pistons I've ever seen had some sort of grey area on the backside of the dome and some sort of triangular logo "cast' into them. The piston numbers were usually stamped into the flat machined area on the dome. Posting a close up pic of the dome and valve notches, and the underside below the pin bosses, might get a better response

I don't plan to pull the heads as this motor ran fine when it was pulled but was in a power wagon with a rear sump pan. I pulled the pan to change it and its more curiosity then necessity that I find out. I'm gonna run this motor either way. I'm curious but not enough to pull the intake and heads. Thanks.

Most of them were TRW for production

Got it figured out. They are in fact old trw flat tops. Thsnks guys