Hi M
Your S looks very much like a Mk1 S
I have got a couple of these and have owned others over the years
All mine have had walnut stocks and were advertised as such
Many Mk 2 S models that I have seen have had beech stocks (walnut stained). There was also a Mk3 version which was very nice
Value depends very much on condition and calibre. The .177's usually go for another £50-75 more than a .22
A ball park figure for a .22 Mk 1 S in nice condition (IMHO)would be £175 or £200-£220 if excellent. Mint condition would attract another £50-£75
John