Hi Julian
According to the London Douglas Motor Cycle Club (LDMCC) Machine Register the machine with frame number YF 2151 is an S6 model i.e. 600cc as opposed to the S5 of 500cc. In case you do not have the outer cover for the primary chain case, The LDMCC pre-war Spares Scheme does have these in stock. The extent of restoration costs will depend upon just how much you can do yourself and the extent of originality you are prepared to seek. These machines were issued with nickel plated barrels. The costs of cleaning, shot blasting, electro stripping of the remains of the old nickel, further shot blasting and chemical cleaning, masking the areas you do not want plating and then electroless nickel plating will be very expensive. Most people simply paint them or in some cases powder coat. They are a very desirable machine and although not rare they do not often change hands. Many consider this model to be one of the nicest early Douglas models with some unique features contributed by the famous Freddie Dixon who is credited with its design. Chris.
