difference in heads both from RD 350

[EE]

Looks like TR3 to me too.

[Dvsrd]

I was concerned that an R5 head on a RD350 would change compression ratio.  The two bikes have different spec compression ratios.

Since Yamaha used "trapped"CR, the difference is influenced by exhaust port height also. The port difference between R5 and RD350 is significant.

[thatguy]

The one on the right came from a stock, 1975 RD350 engine, that, by looking at the OEM, std size pistons, it would be reasonable to assume the head was never off the engine.  But it does not at all resemble either the R5 or the RD350 head I have seen before.  Seems to have less porting for heat dissipation.  Why??????

It's never safe to assume parts are original on a 44 year old motorcycle.