Locomotives with piston valves typically have the eccentrics positioned as in the first of your two K-27 links (the one on top) above.
The link on the bottom shows the other choice, which you'd see with slide valves.
On the Mallet, since it has piston valves, you'd expect the eccentrics to match the top photo.... all four of them.
Now, as David Fletcher pointed out, there are exceptions to this rule ... most involving the positioning of the gear that makes the sliding part of the link block move up and down. But the Mallet, at least as the model is built, is not one of those exceptions, since it matches the construction of Weyerhauser #110 very closely (and the 1:20.me review illustrates that.... so I'm referring to that as a source here.... just in case folks still want to argue about copyright!)
The point is, when the link block is moved down, the eccentric moves in synch with the radius rod, and the locomotive moves forward. When the link block is raised, the link acts as a rocker arm, and reverses the action of the eccentric rod, causing the radius rod to move exactly opposite from the eccentric rod, and the locomotive moves in reverse. If your eccentric rods are backwards, it'll all still work, except that you'll have to move the reversing lever backwards to go forwards and vice versa. If everything else matches, and you have one leading and one trailing, one part of your locomotive will be trying to move forward, and the other will be trying to move backward.... which doesn't work on a locomotive that actually uses its pistons and valve gear to generate power and motion.
Does it matter in an electric model locomotive? Probably not. There are Aristocraft locomotives from a while back that everyone loves a lot that have all that hardware cast in plastic. Old Lionel stuff often doesn't even have all the pieces... and they've been favorites of people for a long time. But, if I'm going to spend a lot of money on a finely detailed model, I'd like it to be right. Notice that with the K-27 above, one of those is right, and the other ... is not. Which would you select, given the choice?
Now ... there is a separate issue involving the binding of valve gear components based on their position. As far as I know, there has not been any published information on what the source of the problem is, or how to best go about addressing it, but there have been pictures posted of valve gear on the Mallet that is, to say the least, "out of whack." I have not heard of any such issue on the K-27, beyond needing to have the eccentrics reversed, which mine have been. So I'll be watching to see when my Mallet arrives, because I want it to run well, and because, well, it matters to me that the valve gear is arranged correctly.
To those to whom it does not matter ... good health, and happy running.