Interesting question. Some reading shows that the very early BN1's, some 20 prototypes were built by Healey at Warwick. The first real production cars were built at Austin Longbridge starting May 1953, with car #138031.
I have a "Autosport" road test dated Oct. 24, 1952 that shows the (O/15) type radiator & the early peaked front grill top.
Another road test, "Motor" dated Oct. 29, 1952 shows the (O/15) type radiator & the early peaked front grill top. Maybe the same car.
A third road test, Motor" Sept. 1953 also shows the (O/15) type radiator but with rounded grill top. Sept. 1953 would have been after the start of Longbridge production, somewhere after car #140205. I can't tell if this test was a Longbridge car with early radiator or a Sept. 1953 road test of an earlier car.
There is a possibility that some of the road tests used some "file" photos that weren't representative of the actual cars being tested.
All other tests & pics show the newer (B/1) type radiator.
My conclusion would be that the early (O/15) radiator was adapted from another car for the prototypes & maybe the first part of production. Since it was likely adapted from an earlier existing car, maybe the Austin A90, the radiator could have well been used as far back as 1950 or so. Note that it does not have a pressure cap vent system. Lots of mysteries.
D