For centuries (at least since Ben Franklin designed early bank notes), the first defense against any sort of counterfeiting has been to introduce a level of complexity that is difficult to duplicate.

When the concern is mechanical duplication, the original needs to be something that a machine can't readily make: smooth, flowing strokes are outside the usual mien of mechanical measurement and machine cutting. High resolution hatching (like the shading on American currency) or other microscale features can defeat photographic attempts, if they can be made finer than the combined resolution of lens and recording medium.

Likely the hardest to counterfeit while still within Victorian technoloy, however, would be a seal with the above features, plus a variable magnetic field across its surface, and wax with fine particles of iron oxide or similar magnetic compound -- particles which would, as the melted wax solidifies under the (relatively) cold seal, preserve the magnetic pattern of the original seal. A "reader" glass containing a thin layer of viscous liquid holding similar magnetic particles could then be easily used to verify the seal on a document without need to have access to the original for this step. The magnetic pattern should be finely detailed as well, ideally complex, and of course unique.

For centuries (at least since Ben Franklin designed early bank notes), the first defense against any sort of counterfeiting has been to introduce a level of complexity that is difficult to duplicate.

When the concern is mechanical duplication, the original needs to be something that a machine can't readily make: smooth, flowing strokes are outside the usual mien of mechanical measurement and machine cutting. High resolution hatching (like the shading on American currency) or other microscale features can defeat photographic attempts, if they can be made finer than the combined resolution of lens and recording medium.

Likely the hardest to counterfeit while still within Victorian technology, however, would be a seal with the above features, plus a variable magnetic field across its surface, and wax with fine particles of iron oxide or similar magnetic compound -- particles which would, as the melted wax solidifies under the (relatively) cold seal, preserve the magnetic pattern of the original seal. A "reader" glass containing a thin layer of viscous liquid holding similar magnetic particles could then be easily used to verify the seal on a document without need to have access to the original for this step. The magnetic pattern should be finely detailed as well, ideally complex, and of course unique.