Webley Service barrels are sometimes found without any serial numbers.
If these are genuine 'Webley' barrels, they have usually been ordered as spares from the factory by retailers, and supplied to customers as and when required.
Barrels ordered for a specific rifle could be marked at the factory with the appropriate serial number, and often were.
When Service rifles are offered with three barrels, it is not uncommon to find differently numbered barrels in the set.
The cased example in my sig. site has similarly numbered barrels.