Would think the boiler sightglass end should be at the firebox door/burner end...
Otherwise it´s as logic would dictate, if solely comparing to the 75 mk.I.
Personally i think that it might just as well be, that the 35-A in fact was intended to stay as any other 35, only with heater/insulation swapped out for ventilation holes/burner.
The 35 version available in the late 50´s, was already then an older well known model, and taking those parts away would alter the detailing/look quite a bit.
That would leave Jensen with two very different looking engines, carrying the same modelnumber at the same time...would have been a first for them, as far as i can recall.
Anyway, all speculative until some proper documentation turns up. Deliberately say documentation, as it wouldn´t take much to fake one of the versions mentioned above.