In 1971 I was stationed at U.S. Naval Station in Newfoundland. I ordered a car thru the Navy exchange, no tax or shipping.
I tried to order a 71 hemi roadrunner, they said that they would try,but I was near the cutoff date for ordering, that I might have to get a 72 roadrunner. I paid $3,127.00. When the car arrived, It was a 72 roadrunner/gtx, blue with white strobe stripe that ran above rear wheels and over roof. Hood scoops had white 440. It was a 440, 4spd pistol grip, bench seat, dana 60 with 3:90 gears suregrip,power steering/brakes, no a/c. It ran a best of 14.2/102 mph in 1/4 with street tires. The engine was orange and had a huge holley carb. I've only seen 2 other 72's and they had 400 ci not 440, and their engines were blue, and their carbs where not holley and looked small. A friend at the local plymouth shop installed a black box from an autotrans car, because it did not have a rev limiter as my 4spd did. He said it had the same trans,driveshaft,dana 60 as a hemi. He never saw another 72 roadrunner with a orange engine, the others were all blue, he thinks I had a 71 with 72 headlights/taillights? I wish I still had the car, because I can't afford one now.  

From everything I've seen/learned over the years, I'll bet it was a true 72, but one of the first 72s down the assembly line, built with a "left-over" 71 engine; the rest of the powertrain was still available, so it went in, too.  I'd bet that they tried to fill your order, but the Hemis were gone, so you got the next best thing.  I have seen many cars built at model-year-change time that had some carryover of parts like that.  

Back in the late 70s, I personally knew of a 72 Road Runner/GTX that was a factory 440+6 barrel car...again, it was an early-built car, built no more than a week or two after the official change-over from 71s to 72s.  It got a 440 6-pack!  The 6-pack was killed at the end of 71 production, but there it was, in full factory trim and chalk markings, (when nobody cared about those details) under the air grabber hood, straight from the dealer.  Everything else about it was pure 72.  It was solid blue, no stripes, with the air grabber.  It happened, but not often.  Yes, you were lucky!  

Likely it was as you described...Ma Mopar tended to use what parts were on hand when a car came down the assembly line....that's how a "few" 440+6's were installed in very-early-production '72s.

Also, the 4-barrel 440 was a regular-production option in the Road Runner through '74....and by checking that on the order blank, you got a "Road Runner GTX," which was Plymouth's name for the 440-powered Bird then, as well as a way to (try to) use up the leftover "GTX" nameplates they still had in their parts inventory. (Also, in '72, the 383 got a displacement boost to 400.)