Fortunately most skilled craftsman take enough pride in their work not to pass it off as anything but their own,
there will always be unscrupulous people who whether they make it themselves or get it after the fact will age an item and pass it off as the real thing.
Sometimes there is a fine line between a "restoration" and reproduction or an out and out fake.
A restoration should always be represented as such, weather it be a repair or replacement of parts,
too many times we see dealers who take a basket case gun replace components with original or reproduction parts to enhance it's value,
when doing restoration work it may be a good idea for the restorer to place a date mark or recognizable restorers stamp on
the new or reproduction parts to limit the options of the faker down the road.
Years ago a fellow brought me an 1866 Winchester that was missing both side plates,
I machined two new ones milled the loading slot and fitted and matched them to the gun,
I also added my initials and date inside where if removed would be obvious that they were replacement.
The customer was annoyed with this arrangement but since it was the only way I would do the work he had no choice or find someone else to do it.
It is sometimes harder to add marks to items like a reconversion from percussion to flint but it can be done. woodwork is another issue.
I know of a case where a man had an original Austrian 58 cal rifled musket that had a small "VMI" carved along the lock mortise,
in "cleaning" the gun he almost obliterated the marking, in his zeal to repair the area around the lock ended up with multiple "VMI"s !
He ruined a historical item and corrupted it in an effort of enhancing its value.
I haven't done any restoration work in the past 15 years,
I may begin to do some as time permits, but I will be very selective of any projects I take on.
Sorry for going so far off topic but this is an topic I feel strongly about.