I think I got a reasonable deal on a early barrel. Just trying to see if it fits the marking designations I'm finding.

M or W between the FSB and a C near the muzzle. Smooth FSB

Tnx for the help!

EDIT:
Just got the variation guide to FINALLY load pics. Suppose its not as early as once thought C marked replaces the 12 and was 67 ish and its deffinately a M not a W

It has an M between the FSB and a C near the muzzle?

Yes

Pics?

Is it an M alone or the standard MP marking?

Alone M believe it predates the C MP C barrel

You need to be looking for a cast FSB.

It looks like an MP C barrel with a very weak P

I saw a really nice 601 1/14 twist barrel today, complete with cast front sight, but he wanted $1900 for it - ouch.

Its not an MP its an M and C at the muzzle 1968 with flash removed on fsb. What I get for jumping when on my phone. But still early barrel for 350 inst bad as long as its not shot out

The Original Colt 601 utilized a Winchester manufactured barrel that was 1:14 Twist. The marking was a very small  "W" between the FSB rings, right side of barrel. Sometimes the W was stamped sideways at times. Colt did not build barrels in house until the enhanced 1:12 twist barrel came along. There is some fuzzy history around the 601/602 transition, as some believe the last 601's built at Colt utilized the new 1:12 twist. This may be within the last several hundred or one thousand guns.The Cast FSB went away in favor of the forged with the next model. Try not to get hung up on all of the accuracy of stampings on barrels due to the fact that it was inconsistent at best from day to day on the Colt production lines. It is also believed that after Project Agile was over in Vietnam (the first combat testing), the rifles were returned to Colt, and re-fitted with the enhanced 1:12 Twist barrels, and returned to service.

What about the 1/14 601 barrels with the VP between the front sight legs?  I thought those WERE produced in house by Colt. Also, if you start accepting "replacement" barrels as being "correct" for a particular model, then wouldn't ANY military barrel now be right for ANY build?

They were. Check The Black Rifle page 97

I agree with this. ^^^ That P is very light.

The style of M on the OP's barrel is different than the M of a 601 barrel & the same as the MP markings. In the OP's pic you can see the bottom portion of the P sticking out from the M.

From Ekie's guide regarding the FSB machined smooth & drain hole: "the forged front sight base was machined smooth to remove forging flash and used on M16 and XM16E1 rifles, and on the M16A1 for a time. Production started as early as 1964, and was used as late as 1969 and serial number 1,750,XXX. A bottom drain hole for the front sight post assy was added in 1967"

A slight correction:  During the early 601 production, Colt did not have the equipment to produce the barrels so they contracted that job to Winchester.  Before the 1-12, however, Colt got up to speed and started producing their own 1-14 barrels.
You can find original "Model 01" rifles, then, with Winchester-made 1-14, Colt-made 1-14 and Colt-made 1-12 barrels.

The simplest tell if a 1-14 barrel was built by Colt is the "C" stamped on the rim of the barrel extension (pic below), but they are also slightly thicker forward of the front sight.  
That section between the FSB and the FH is not "straight," but if you take three measurements (close to FSB, close to FH and half-way between them) and average them, you'll get a "measurement" for a particular barrel.  
According to Ekie's guide, the Winchester barrels showed a diameter in that area of "about .565"" and Colt-manufactured barrels ran about "about .575"."  Based on the very few samples I've been able to measure, however, I believe the Colt barrels ran as light as .570".



As for markings, 1-14 barrels came with a huge variety of stamps--or with no markings at all.  Worse, the presence of a "Colt" mark doesn't absolutely prove Colt manufacture.  The pic below is of the barrel on Model 01 #000748.  It is a Winchester-manufactured barrel but carries a Colt "verified proof" stamp, which leads me to believe Colt did the proof testing on barrels they received from Winchester.