I pulled off a straight back 2001 Bravo III and bought a recently rebuilt 2006+ swept back version. I was verifying ratio by turning input shaft on the new drive, and I found that it only turns the output shaft when span LH rotation. Spinning input shaft RH, it wouldn't turn output shaft. Is this normal? The straight back bravo III will turn the output shaft when spinning input shaft LH, and then opposite rotation on the output shaft when spinning the input shaft RH.

What's the difference in these version Bravo III's to do that?

The older unit has a stuck clutch.

When working correctly,( and in gear) the propshafts should only turn when the
input shaft is rotated CCW.( like standard rotation engines rotate)

So what is the side effect of the clutch being stuck? I do find that when the older drive is put in neutral, the output does not turn. So seems to be normal in neutral, but both in reverse and forward, will turn both ways.

Old/New upper should turn both directions when in either FWD or REV. Something strange is going on with the one you have. If it was just partially in gear (clutch didn't slide all the way) there should at least be some clicking of the gears

This unit I bought had new shaft, clutch and gear set put in 3 yrs ago. Was a spare drive and never ran. What’s the chances running this and break in would resolve this issue?

I wouldn't run it until I could figure out why its doing it.
When the shift fork is moved to FWD or REV the yoke and cam assembly is tensioned to move up or down from neutral. Its not a direct correlation between the fork moving and it going into gear. As the upper shaft rotates it "should" move in the given direction. It may not because the clutch and clutch shaft are binding.

A "new" assembly should have no issues moving if the upper was put back together correctly. Only reason it would not move is something is not to spec, incorrect lube. When the shaft and clutch get worn folks talk about having to give the motor more throttle and then it slams in gear.

If the output shaft doesn’t turn when it’s in neutral it’s ok,
the drive must be loosened up enough that there’s enough drag
produced by the garter springs ( pushing on the clutch)to rotate the vertical shaft.

try holding the prop shaft while you turn the input shaft RH, it shouldn’t turn

The way these things work involves the spiral splines on the clutch shaft and
the cone clutch that has those same splines( female)…. It’s like nut ( cone clutch)
and a bolt ( clutch shaft)….. one way it tightens up, the opposite way it loosens

When shifted, the cone moves into the tapered cup on the gear, the spinning gear
drives the cone down the clutch shaft til the shoulder on the clutch shaft pinches against the bottom of the driven gear,
the more resistance from the veritical shaft, the harder the cone is driven into the cup

If you rotate the gear ( via the input shaft) the opposite direction,none of this magic happens

So I tried holding the prop shaft while turning RH, and I can stall the prop shaft even while spinning input shaft. If I rotate LH, I can not stall the prop shaft. This is on the older drive. The newwer drive is already installed. That drive was pretty tight yet when trying to spin input shaft in either direction. I’ll assume with some break in, it will change.