Is the factory high compression piston flat or domed?

I know that the MP pistons are domed.. I dont know what you mean
with "factory high comp"

By "factory high compression" I assume you mean '68-'71 OE stuff. Flat tops with valve relief eyebrows.

1970 340

Early 340 pistons had a slight dome and required a different Edelbrock head for it to clear. In 1972 the compression dropped

What am I looking at on the cyl wall of mainly on 4 and 6.. is that some
sort of reflection or what

Is it just like the factory 10.5 piston style that was used for the 273? 2 valve reliefs with a little dome.

I thought they were a flat top with the eyebrows, .020" above deck, but no dome?

Technically a flat top dome with a small chamfer that sticks above the block. I used to have a couple of sets but I must have sold them.

Having started racing 71 and 72 340's in stock eliminator back when stock factory pistons were required I have a few sets. The 71 pistons at blueprint specs were .045 above deck with an .028 gasket and had 2 valve notches.The 72 and later pistons were .054 down in the block with a .032 gasket and 4 notches. With the 71 at specs and the head at 63.3 cc's the eyebrow above the intake valve would kiss the piston and the valve notches were not deep enough for the cam. It took some massaging to get everything to fit.

The main drawback with the early 340 piston is the weight........seriously heavy !

Even the low comp pistons are heavy. Only the 360 had light pistons.

That what i have seen with the 340s i have taken apart. I have factory 273 pistons, they nothing like a 340 piston.

You guys are talking factory stock pistons.. all I ever use for my 340s are
aftermarket pistons.. I use nothing except the block and MAYBE the crank
every thing else is aftermarket or MP stuff

I found, on my shelf, some forged , standard bore 340 pistons. The box said flat top. Don't have a part# so I'm just trying to see what the factory piston looked like and what my cylinder head options might be.

That is going to depend on where they are going to end up in relation to the deck. Above, flush, or in the hole. You can't tell that by looking at them, you need to measure pin center to top and do some math. If they are above deck, the head design needs to accommodate that.