Are you talking about the hidden secondary HIN numbers that the manufacturer is supposed to stamp somewhere inside the hull, inconspicuously?
If that's not what you are talking about, then please ignore the following:
This requirement was put in force to aid in identifying a boat that has been stolen and recovered. The manufacturers are not supposed to reveal where these numbers are hidden, as that would destroy the benefit of these secret numbers. So you should not be able to see these numbers at all.
For shoebox-type production, the Coast Guard recommends to Mfg's put them somewhere under the lip of the hull/deck joint, before the hull and deck are put together, so one would have to pull that deck piece off to see it.
Boats manufactured or imported on or after August 1, 1984, also have a duplicate secondary HIN affixed somewhere on an unexposed location inside the boat or beneath a fitting or item of hardware. The purpose is to help authorities identify your boat if a thief or vandals remove or damage the primary HIN on the transom. It is illegal for anyone (manufacturer, dealer, distributor, or owner) to alter or remove a HIN without the express written authorization of the Commandant, U.S. Coast Guard