As someone already stated, not wanting to get between the crossed swords of Bill and Mike, however, S&W as a matter of policy does not typically record the completion dates of their guns. We have all come to accept that Roy's list of shipping dates is the S&W manufacture date and has been accepted by the ATF for their purposes.
S&W has not regularly shipped in serial number sequence.
That said, and I don't recall which gun this applies to, the gun that was shipped first would have to get my vote for the first Combat Magnum as far as S&W's are concerned.
Had 260001 and 260003 sat in the vault for several months and 260002 shipped before them, we would all be in agreement that 260002 was the "first" Combat Magnum.
This is merely my humble opinion and its worth exactly what you paid for it.
Any objections, complaints or pending lawsuits should be directed to my attorneys at Dewey, Screwum and Howe.