I agree with you that this could be said to be a precursor of the Lagonda Rapide,but I have it from 1955,so I think that this one is the prototype or the first Lagonda Rapide built by Tickford and owned by David Brown.
"The 'Brown Bomber' was a Lagonda built on a tubular chassis much the same as the Aston-Martin DB 111 and DB 111 S's. It was intended to be fitted with the V12 engine under development at that time but the engine was quite a failure due to the method of locating the crankshaft but that is another story.
After the Experimental Department had finished with it, the 'Brown Bomber' was passed over to the Service Department where it was extensively used by David Brown who was driven by his chauffeur George Walters. It had a 3-litre LBS engine which we ported and fitted some illegal camshafts etc to give it a bit more power. It was an extremely large car so was under powered with a 3-litre engine. Four people could sit side by side on the front bench seat and at that time it had the largest curved windscreen ever produced by Triplex. Whatever possessed the management to scrap it before the move to Newport Pagnell in 1963 was a disgrace! At that time, not much thought was given to the historical value of such vehicles"
I believe that this one is the Brown Bomber