'20mm' bore? If it's an English shotgun, made around 1840 - 1850, judging by the back-action-style lock, then it won't have a metric-measured bore. 18mm is 12 gauge, by the way. Here in UK we STILL don't mark our domestically-made shotguns in metric measurements.
All the proof marks will be found underneath the breech-end of the barrel - it also appears to have a hooked breech, meaning that once you've removed the single key, by a few gentle taps, it ought to come out easily by lifting the muzzle.
Here is a list of Worcester and Worcestershire gunmakers of the 19th century - as you can see, William Wood of Worcester is the last name in the list - it took me almost fifteen seconds to find it..........note that the town of Hales Owen is now called Halesowen, saving a lot of ink, no doubt.
Alan & Thumber Worcester
Allen & Thurber Worcester
E C Asthorpe Unknown address Severn Stoke
Benjamin Attwood Hayseech, Old Hill Hales Owen
Averill & Son Evesham
J C Banfield & Sons Tenbury Wells
Rowland Barnsley Hales Owen
Simon (Samuel) Benson Salop Street Dudley
Birmingham Gun Barrel Co Ltd Hayseech Old Hill Halesowen
William Bourne Droitwich
Samuel Brown Summer Hill Hales Owen
William Burgess Malvern Wells Malvern
Alfred & Arthur Richard Burr Hayseech Hales Owen
John Burr Hayseech Mill Hales Owen
Thomas Droman Dudley Road Halesowen
Thomas Drummond Dudley Road Halesowen
Edge & Rose Birch Hill Mill Hales Owen
John & Abraham Edmunds Cornmarket Worcester
Joseph Fellows Stomber Mill Stourbridge
James Fletcher Bridge Street Evesham
A J Foster Unknown address Kidderminster
John Gill 13 New Street Worcester
L Edwin Goff Dodford Bromsgrove
William Griffiths 5 Bridge Street Worcester
James Hawker Coventry Street Kidderminster
Harry Higgins 46-48 Teme Street Tenbury Wells
Higgs & Price Hayseech Hales Owen
Hughes-Johnson Stamping Ltd Langley Engineering Works Oldbury
James Humpherson High Street Bewdley
John Lambert 85 Sidbury Worcester
Thomas Lane 4 Corn Street / Corn Market Worcester
Edward Male Worcester Street Bromsgrove
John Osborne Bridge Street Evesham
John Perkins New Street Worcester
John Perrins College Street Worcester
Thomas Phipps Evesham
H E Pollard 61 Broad Street Worcester
Joseph Price New Town, Netherton - See Price & Hibbert Dudley
Price & Hibbert New Town, Netherton Dudley
William & Major Rose See Aaron Rose Halesowen
Aaron Rose Birch Hill Mill Hales Owen
John Rose & Co Crown Gun Barrel Works, near Coombs Wood Hales Owen
Rose Brothers See Aaron Rose Halesowen
Rose Tube Co. Ltd See Aaron Rose Hales Owen
James Sawyer Hawne Lane Hales Owen
Robert Scott 31 College Street Worcester
John Shaw 16 Moore Street Worcester
Arthur Joseph Simpson Crabbs Cross Redditch
Charles & John Siviter Runbow Hales Owen
John Southall Bromsgrove
John Stanton Coventry Street Kidderminster
Thomas Stephens Bridge Street Pershore
J T Taylor Bromsgrove
T Thacker & Co 62 Broad Street Worcester
William Tromans Whitehall Hales Owen
Thomas Trueman / Truman Dudley Road Hales Owen
Ward & Son Worcester
Whiteman Brothers Ltd Silver Street Worcester
Richard Wise Kidderminster
William Wood 18 Broad Street Worcester