Taken from the website:
As you would know the title BN generally meant two seater. The BN1 and 2 had two seats, BN6 and BN7 also only had two seats, but somehow I suspect that the use of the BN title such as this didn’t really commence until the 100/6 BN6. After all the BN4 was a four seater. Avid readers of the Austin-Healey publications that abound these days would have realised that there were also a BN3 and BN5. There was one BN5, a 4 seater used as a mule by the Donald Healey Motor Company (DHMC) for the 1957 Mille Miglia with disc wheels and detuned engine and was eventually converted to BN4 specifications and sold. There were also at least two BN3s built by the DHMC as experimental prototype vehicles to test the new six cylinder in the Austin-Healey chassis. One of these, BN3/4 which still exists in terrible condition was a very different car being nine inches longer then standard, special chassis that sweeps up into the engine bay and quarter elliptics at the rear. The other vehicle is the BN3/1 and is the car I bought over a quarter of a century ago.