Here is a quote from the esteemed Mr. O'Conner:
" For those used to the military sling and prefering the two-piece jobs, sporting slings like the military sling but made of lighter and narrower leather straps (7/8 to 1 inch) are available. Still better is the one-piece sling worked out by Colonel Townsend Whelen, dean of the American gun writers, back before World War I. This one-piece Whelen sling is a strip of leather from 3/4 to 1 inch wide and generally about 52 inches long. It has a claw hook at one end and the length for carrying is adjusted by the placement of the claw hook in a series of holes punched in the leather. It likewise has two leather "keepers." The sling is held together and the size of the "loop" is regulated by leather lacings which tie through the holes.
I have used the one-piece Whelen sling for many years and on the whole have found it very satisfactory. I adjust it so that it is of the proper length to carry the rifle with the sling over my right shoulder with the trigger guard forward. I hold the strap with my right hand to steady it. If it is necessary for me to sling the rifle over my backso that I can use both hands for climbing, carrrying out a couple of quarters of venison, a sheep head, or some other burden, I simply move the claw hook back to lengthen the sling."
I use a Montana sling that sort of does the same thing. I've never seen a Whelen sling.
Maybe the 1907 and Whelen sling are so close as to not matter. I've got a couple of the Turners 1907's and a Brownells 1907. All work excellent. I'd recommend the upgraded Grovetec swivels. Very robust metal swivels that will not fail.
Another to take a look at is the Brownells Latigo. It does the same as the 1907's but is supposed to be a touch easier to adjust.
The Whelen sling is a more simple way of using a shooting sling (not carry sling). The top end is set so the sling creates the third side of the triangle to steady your left arm when your in a shooting position. You put your forearm through the loop in the sling just as with a 1907. You can use the top keeper to pull up the tension so its snug on your upper arm. You can also create the tension quickly without placing your arm through the loop. The tag end also has a claw so that you can set it for a comfortable carry length. The second keeper can be used to keep the sling in the swivel if you need to detach the tag end claw if the carry position makes the sling too short for your shooting position. The only difference is the 1907 uses two connecting leather straps and the Whelen only uses one.
If you like the military shooting sling format the Brownells Latigo is, as mentioned previously, a great way to go. I have a couple of the 1907s on heavier varmint rifles that often wear bipods because, while I love Col Whelen's shooting method they're on the heavy side for a big game rifle.
I've gravitated toward the Montana sling which is similar to the Latigo but a bit lighter in weight: