Generally Blackbirds follow the models used by the main brand, of which they share the colour numbering and part of the numbering scheme, but are distinguished by the prefix BB, and at least initially, by the use of only the unit number corresponding to the nib. These pens are generally produced with the simplest of the available filling systems (mostly lever filler), although in parallel to the model Visofil was also introduced a model, the Blackbird Topfiller, with a similar filling system.
The same brand seems to have been used (as Le Merle) for production destined to French market. There is also  a different name found on a exemplar bearing the engraving: LE MERLE BLANC, MADE BY THE "SWAN" PEN PEOPLE, FABRIQUE EN ANGLETERRE, and the same name, Merle Blanc also appears in the advertising on the side.
- as reported in this topic.