Identification by layout/specially designed imprints
When speaking of “layout” I mean the address side first. All imprints / elements found here, their positions, measurements, the way the writing lines are arranged, stamp box, printing inks used etc. can help to identify a printer. Many printers or better say the composers / people responsible for making ready the address side formes followed their usual routine for a longer period. Also all ‘neutral’ parts of the address side (when done by letterpress) was used over and over again.
Of further interest are the type fonts used for imprints/captions. Some were not as common as others. Keep in mind that the type (for letterpress process) was usually bought from type foundries. So, this is nothing exclusive. I have examples of address side layouts for cards printed by a particular firm at about the same time showing a variety of different fonts used from card to card. Guess they were short of type, under time pressure and used what was still at hand.