What is it?

[Brandis]

The round thing below the oil tank.  Perhaps only on the first 200 with the pogo stand, which I have.

[CheeserBeezer]

It's the horn!

[Greybeard]

That's where the spare dilithium crystals are stored.

[RDfella]

Spare gas mask filter .....

[Brandis]

Well, that was infinitely stupid of me.

Not where I expected it.   Let's see.  Where did I put my dunce cap?   

[a10 gf]

(Late to the show) Ain't it the container for round biscuits, snacks for the trip ?

But seriously, would be nice to get any details and info about that horn, rare (Lucas?) things always interesting.

[groily]

Looks like a Lucas HF1440 from the drawing maybe, but it's not that clear.

Lucas parts lists say HF1234 or 1235 for all A7s and 10s from the beginning (and for most other BSAs supplied with a horn as far as I can see from a quick look), but the lists sometimes seem to omit exceptions or export specs - or maybe parts bins needed using up.
(Lucas Workshop Instructions Vol 2 Section L7 parts A and B Issue 1 from June 1953 has the info on both series)

[RichardL]

It's a horn from a tugboat (he said, with sincere respect).

Richard L.

Edit: So you don't think I'm crazy, I know it's not really from a tugboat.