There are no markings on either the Douglas or Wilson barrel that indicate who made it. The non prefix #1s and on up through ABOUT 130-05000 should have Douglas barrels BUT as we all know they weren't made/assembled/shipped in any kind of order so the closer you get to 130-5000 the less likely it is to be a Douglas barrel. There is no data at the factory that can tell you what barrel was on a specific rifle.
Without knowing the serial number of the receiver it came off of it's not even possible to make an educated guess. You say an "EARLY pre warning "A" barrel. There are probably fewer than 10 non prefix 270s in "A" configuration. So what it gets down to is how many "A"s in 270 Win did they make in 1970, 1971 and 1972? That is compared to 1973 through mid 1977 when they started adding the warning to the barrel. They made approximately 3000 TOTAL #1s in 1970-1972 only a small portion of which were "A"s
Between 1973 through mid 1977 when the warning began to appear they made approx 40,000 #1s over half of which were made in 1976. Whatever % you want to use as being "A"s and assign to each group it would be highly suggestive that in all likelyhood it's a Wilson barrel. The chances are certainly FAR higher than it being a Douglas barrel.
That said, UNLESS you are restoring one it shouldn't matter.
I own #1s with Douglas barrels, Wilson Barrels and Ruger barrels along with custom barrels from Douglas, Shilen, Hart, Kreiger, Llija and several others and frankly I can't tell the difference.
The only Wilson barrels I've had issues with in #1s were the very early ones in specific cartridges, the 7x57 for one which had REALLY long throats. My 7x57 "A" made/shipped in 1977 has such a long throat it is impossible to load 120s or 140s any where near the lands without the bullet simply not seating in the case.
On the other hand one of the MOST accurate #1s I own is a 22BR that started life as a 218 Bee in "S" configuration. It was set back two threads, rechambered, the extractor recut and all the rest but it is simply a factory Wilson barrel with the original factory sporter crown. To date I have NEVER got my factory 22PPC "V" with a Ruger barrel to shoot as well. The ONLY #1 I have that will outshoot that messed with Wilson barrel is a 1970, very early 130 that I pulled the 22-250 "V" barrel on and had Dog Shilen install a Match Grade 257 Roberts "V" barrel on throated specifically for the 75 grain Sierra HP with a tight neck and a chamber cut at minimum etc.
I own numerous 22-250 "B"s in non prefix (Douglas) very early 130 (Douglas), 130-08xxx Wilson) and a 132 that should be Ruger. In fact in the right light you can actually see the hammer forging. They all shoot about the same, sub MOA and I certainly can't claim the Douglas barrels are any more accurate than the Wilson or Ruger barrels and frankly all the custom barrels with the exception of one Shilen fall within a spread covered by factory barrels.
I think I'd simply install it, shoot it and enjoy it. ;-)
SOME Ruger made barrels will show the exterior hammer forging marks when held at an oblique angle to a bright light source. Frankly I wished they would leave them like the do on the Target 10-22. The hammer forging certainly looks good on my Styer. ;-)
The best
Ross
