Those Pulsometer pumps are described as "immersed" but only the uppermost flange was actually set into the fuel tank - hence the teardrop fairings below them. Attached is a close-up of a drawing similar to Greg's (albeit of a B.Mk.II) showing the arrangement of the Pulsometer FB fuel pumps.
Apparently some B.Mk.Is lacked those fuel pumps on the inboard, 'No. 1' 580 Imp. Gallon tanks. This was 'Mod. 594 (temporary)' which substituted stack pipes on the No. 1 tanks but retained pumps on the other two sets of outboard tanks.
There is also mention of alternative Thomson fuel pumps but, I believe, only on Victory-built Mk.X Lancs. A look at the underside of Mk.Xs shows the same (or similar?) teardrop bulges as on UK-built Lancs.
Pulsometer refers, of course, to the London-based Pulsometer Engineering Company, Ltd. I'm less certain about who "Thomson" was. At first, I assumed that they were US or Canadian ... but now I'm wondering if it might be Thompson Bros. (Bilston), Ltd - the maker's of fuel bowsers? British Thomson-Houston also occurred to me (but then, I suspect, the pumps would be dubbed 'BTH' rather than 'Thomson').