Leon,
Only nearly 10 years late with some more serial numbers for you
My list so far from LDMCC Machine register, your sightings, other sources and my sightings.
Frames (HF)
105, 108, 112, 258, 263, 320, 345, 391, 399, 437, 627, 692, 707, 733, 766, 790, 806, 823, 831, 833, 835, 836, 863, 913, 934
Engines (OE)
106,118, 124, 237, 249, 271, 273, 323, 338, 372, 399, 425, 530, 566, 679, 739, 752, 758, 770, 780, 807, 822, 824, 830, 836, 839, 842, 892, 896, 928, 943
Gearboxes (PG)
203, 212, 384, 386, 407, 415, 646, 701, 733, 747, 752, 792, 798, 806, 830, 844, 845, 851, 859, 861, N864, 893
From this I suspect no gap in the serial numbers of OB's? My guess is about 850-900 OB's made. Not many survivors and yes I agree, they are a bit thin in the 400-600 serial number region ! Maybe more of the early and late ones escaped the speedway somehow?
Cheers
Hutch