Here is the fixing reference for just about every nut & bolt for the Lambretta courtesy of Dapper. (Originally posted March 2010 on the old blog)

LAMBRETTA fixings list
1. Horn Casting

M5 x 10mm Pan Head Slotted Screw (2 Required)
M6 x 18mm Allen Head Screw + 2x6mm Conical Washers (2 Sets Required)
M5 x 20mm Countersunk Oval Slotted Screw + Finishing Washer (2 Sets Required)
M5 x 16mm Hex Head Bolt + 5mm Large Diameter Conical Washer (2 Sets Required)
M4 x 10mm Countersunk Oval Head Slotted Screw (Oval is optional, just looks better) (2 Required)

2. Front Mudguard

M5 x 25mm Countersunk Flat Top Slotted Screw + 5mm Nut, 5mm Conical Washer (2 Sets Required)
M5 x 20mm Countersunk Oval Slotted Screw + Finishing Washer, 5mm Nut, 5mm Conical Washer (2 Sets Required)

3. Floor Set inc. Bridge Plate

M4 x 10mm Pan Head Screw + 4mm Speed Nut, and
M4 x 20mm Pan Head Screw + 4mm Nut, 4mm Large Diameter Washer
M4 x 10mm Pan Head Screw + 4mm Speed Nut (2 Sets required)
(In lieu of speed nuts, you can use plain nuts and washers)
M4 x 20mm Countersunk Oval Slotted Screw + 4mm Nuts, 4mm Washers (4 Sets Required)
M4 x 25mm Countersunk Oval Slotted Screw + 4 mm Nuts, 4mm Large Diameter Washers (2 Sets Required)
M4 x 10 Countersunk Oval Slotted Screw = Finishing Nut (2 Sets Required)
14 x 5mm Nuts
14 x 5mm Conical Washers
10 x 5mm Speed Nuts
M5 x 25 Countersunk Oval Slotted Screw + 5mm Nut, 5mm Conical Washer

4. Dual Seat

M8 x 25mm Hex Head Bolt + 8mm Nut, 8mm Washer (8 Sets Required)
4 x 8mm Flat Metal Washer
Large Diameter 8mm Plastic Washer. (4 Required)

5. Frame ‘L’ Bracket

M7 x 20mm Hex Head Bolt + 7mm Nut, 7mm Conical Washer (2 Sets Required)

6. Engine Cowlings

M5 x 10mm Slotted Hex Head Bolt + 5mm Lock Washer (7 Required)
M8 x 14mm Hex Head Bolt + 8mm Conical Washer (1 Required)

7. Headset

M5 x 40mm Countersunk Oval Slotted Screw + 5mm Countersunk Internal Lock Washer (2 Sets Required)
M6 x 10mm Countersunk Oval Slotted Screw (2 Required)
M3 x 20mm Countersunk Oval Machine Screw (Philips Head) (4 Required)
M3 x 22mm Countersunk Flat Slotted Screw (2 Required)