The original headlamps were Lucas Type RB60, which were of 8 inch diameter (measured across the visible diameter of the glass, i.e. the inside of the rim).  These lamps had concave spherical mounting recesses that fitted the cast brackets clamped onto the radiator tie-bar.  The RB60 came in several variants, some with thick rims, some with thin ones, some with a tripod for the bulb holder, some with the bulb holder at the rear of the reflector.  The lamps found on original FWD cars seem to have had the thick rims, which were secured onto three pegs fitted in the shells by pushing and turning like a bayonet fitting  (i.e. there were no top hinges, nor securing screws, as on later Lucas headlamps).  Also, the headlamps on FWD cars seem to have had the tripod-type bulb holders.  For identification and comparison, the photos below show both the original pattern headlamps and some other types that would be suitable if the originals are not available.

It should be noted that Lucas made other headlamps that were basically similar to the RB60, but had detail differences.  An example is the R67B, which had a convex mounting dome in place of the concave one.  These shells can be modified  to the right form for the FWD, or you could follow Paul Bamford's example and modify the supporting brackets instead (see photos below).

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This image, showing the headlamps, appears in the FWD Instruction Manual.

A beautifully restored RB60 on Alan Stote's FE type tourer (3 photos).

Note the tripod mounting for the rearward-facing bulb. This appears to have been standard fitment on the FWD.

The "thick" type rim is easily seen in the side view.

An original RB60 on chassis 7231

Photographed in 1995

An original RB60 on chassis 7231

Photographed in 1995

An original RB60, from a FWD Alvis, unrestored.

Note the "thick" rim, and recessed mounting dome.

Same lamp as previous.

Note the "thick" rim, in front view, and the tripod for the bulb socket.

An RB60 found at an autojumble.

This lamp is clearly stamped "RB60" and has the recessed mounting dome, but the rim is "thin" type.

Same lamp as previous, front view of "thin" rim.

Original type headlamp mounting clamps.

R67B headlamp on Paul Bamford's car in 2006. The lamp has the "thin" type rims.

The ribbed glass, cut from shower screen glass, has "miraculous optical qualities", says Bamford.

Mounting of the R67B. Paul decided not to modify the lamp to fit the original Alvis headlamp supports, but to make new supports to suit the lamps.