In the statute quoted below there is only ONE requirement for the certificate to be issued and it is bolded. It says nothing about confiscating a previously unregistered gun.
28.429. Pistols; safety inspection required;
certificate of inspection; exemptions; requirements
of pistol presented for inspection;
violation as civil infraction; fine.
(1) A person within the state who owns or
comes into possession of a pistol shall, if he or
she resides in a city, township, or village having
an organized police department, present the pistol
for safety inspection to the commissioner or
chief of police of the city, township, or village police
department or to a duly authorized deputy of
the commissioner or chief of police. If that person
resides in a part of the county not included
within a city, township, or village having an organized
police department, he or she shall present
the pistol for safety inspection to the sheriff
of the county or to a duly authorized deputy of
the sheriff. If the person presenting the pistol is
eligible to possess a pistol under section 2(1), a
certificate of inspection shall be issued in triplicate
on a form provided by the director of the
department of state police, containing the name,
age, address, description, and signature of the
person presenting the pistol for inspection, together
with a full description of the pistol. The
original of the certificate shall be delivered to the
registrant. The duplicate of the certificate shall
be mailed within 48 hours to the director of the
department of state police and filed and indexed
by the department and kept as a permanent official
record. The triplicate of the certificate shall
be retained and filed in the office of the sheriff,
commissioner, or chief of police. This section
does not apply to a wholesale or retail dealer in
firearms who regularly engages in the business
of selling pistols at retail, or to a person who
holds a collection of pistols kept for the purpose
of display as relics or curios and that are not
made for modern ammunition or are permanently
deactivated.
(2) A person who presents a pistol for a safety
inspection under subsection (1) shall ensure that
the pistol is unloaded and that the pistol is
equipped with a trigger lock or other disabling
mechanism or encased when the pistol is presented
for inspection. A person who violates this
subsection is responsible for a state civil infraction
and may be ordered to pay a civil fine of not
more than $50.00.