So how old would this upper be if it's not an upside dowm "M"?



By the flash hider:



And on the carry handle by the sights:



And by the CH:

It looks like the upper receiver is a 601, no arrow, smooth under the carry handle, octagon profile above the charging handle, thick webbing in back.  (check for Colt proof marks in front of the port door)  The barrel looks like a "M VP" barrel.  (1964 - 1966?)  Has it been re-parked?

EDIT:  It looks like the barrel has been painted over, I see green tint park showing through some worn spots.  (right?)

Boxed "C" and the VP Triangle by front right of upper.  yes, barrel was reparked by one of the Army depot (don't know exactly which one).

ETA: Yes Sir!  repark flaking off.  Looks grey to me.

Very nice.  What kind of hand guards are on it?  Early bend gas tube?  I have a "M VP" barrel that has bee re-parked and looks very similar to yours.  I wasn't sure if the military would have done this or not.  Does anyone have more information about this?

EDIT:  Oh, are you saying it is re-parked or something is flaking off?  

Upper receiver is about as old as they get 601.  Barrel not quite as old still a MVP.  Re-arsenal was not uncommon.  No drain handguards and early bend tube would probably give an idea of when re-arsenal.  Drain hole HG's and later tube would probably make re-arsenal late 60's.  Surprised they didn't ditch the barrel.  Sight wheel and charging handle have been replaced.  Still a pretty nice piece.

What type port door does it have, nobar?

the latest door.....

But it does have a slickside chrome BCG.

That's still a very nice upper assembly!  Looks like you have the two major components for a 601.

Anyone know how I would strip the repark and get to the original grey park on the upper receiver?

I'm a little confused, is there finish coming off the upper receiver?  If so you can try some paint stripper but pay more and buy it from an automotive paint supply shop, hardware store strength paint stripper generally doesn't work as well.  If (automotive) paint stripper is going to work you will see it start to work within a couple minutes.  If stripper doesn't work or you don't like what is under the paint I would contact US Anodizing to have it re-anodized.  (The aluminum upper receiver was anodized during manufacture)  If some kind of finish is coming off the barrel you can try stripper on that too, yours has the "frosty" appearance of that makes me think it has already been re-parked.  I'm not sure who re-parks barrels but I understand that the first part of that process is to (glass bead / sand) blast away the existing finish.

I'll get a pic of the area where the CH meets the upper and you can see where the repark has either flaked or wore off somehow and I can see the original grey coating!  They probably just reparked over the original.

Send both the upper and barrel to Victor at US Anodizing,he will take care of it for you.You will need to dis-assemble it or he charges a bit more