Gentlemen,
Thank all three of you for your replies (and chankly bore for his PM). This is great information, and knowing the original British Standard (i.e. L40) should let me track down its precise composition, heat treatment, physical properties, etc.
My recent interest in this particular question was prompted by an interest in the welding of the wide variety of Al alloys used on motorcycles over the past century. Although I'm not planning to weld my Girdaulics, they're an excellent example of the less common use of Al alloy in a structural element during the "pre-modern" era of motorcycle manufacturing (notwithstanding the use of the entire Vincent engine as such...).
Typically, back in the classic era Al was used simply to reduce weight in non-stressed components like engine cases, gearboxes, timing covers, etc. so the engineers didn't have to put much, if any, thought into optimizing the cross sections of the castings (or forgings) for strength. Most often if a boss is in a casting it is there, not for strength, but because the designer needed it to be thicker in that position in order to place a tapped hole. Obviously, that's not the case with Girdraulics.
