Just about ALL centerfire rifle barrels are marked SB2, what the barrel is marked is irrelevant to what frame it's on, what frame they put them on just depends when it gets made into a rifle, barrels are made in runs of 100-500 by Marlin in Connecticut, then rifles are built in Ilion NY. SB1 357 and 44mags were first available in 2008, were in the 2008 catalog in late 2007, they were initially marketed for the Indiana pistol chambered rifle deer season without sights, they still come that way, the SB2 357 and 44mags are no longer made, haven't been since late 2007/early 2008. Since one member got a 45-70 accessory barrel recently with no sights, they may be changing all of the barrels that used to come with sights to just scope rails, dunno, that remains to be seen.
But if you want a 44mag accessory barrel, be sure to order it the way you want it, but for right now they're out of stock or have been for quite some time. One dead give away to those on SB1 frames is the marking on the right side of the frame where no markings are usually made except for special edition models, a shortened version similar in effect as stated on the website and in the catalog. They should also have the new style synthetic stock.
X= 2007, so I suspect that's an SB2 framed rifle.
Tim
http://www.hr1871.com/Firearms/Rifles/handiRifle.asp*Note: The frame of the 44 Mag. (SB1-S44) and the 357 Mag. (SB1-S35) cannot be fit with any other centerfire rifle barrel. They are capable of being fit with accessory shotgun barrels, but their rifle capability is limited to the specific 44 or 357 Mag. barrel that it is shipped on the gun.