I own one of the 5" 629-3 Classic DX revolvers. It is true that the 5" version was only made for one year, 1994 I believe. Mine is my main hunting handgun.
And in fact test-firing DID qualify a Classic for "DX" status. If you look closely at the laser lettering on the right side of the barrel of a DX, you will see the lettering "629 Classic" is centered on the barrel while adding the "DX" later resulted in making the entire lettering off-center toward the muzzle. Also, the "DX" portion of the lettering on most I've seen does not match the "629 Classic" in darkness, again indicating the "DX" was added post-production.
I've been told by several S&W collectors with more knowledge of them than I possess that the DX guns had their actions massaged but I cannot confirm that. But I also own a 6.5" 629-4 Classic and a 6" 629-1 with the usual half-lug barrel and the DX is the most accurate and has the best trigger. I just wish it had a square butt.
I don't know how many 5" 629 Classic DXs were made but a single year's production of the most accurate guns with one barrel length that was less popular (at that time) than its shorter and longer brethren cannot be a very high number.
Ed