Harkening back to the good old "hell for leather" days of the U.S. cavalry, the saddle ring attachment on Marlin lever action rifles have been around for over a century and is still available (of sorts) today.
Why the saddle ring?
Close up of Marlin 1893 SRC .30-30 Win caliber saddle ring carbine via Collectors Arms
European horse soldiers of the Napoleonic Wars often added a small carbine to their more traditional armament of saddle pistol, saber, heavy sword, and lance. At some point, a clever fellow figured out that the easiest way to carry these short rifles was to sling a strap across their chest from shoulder to waist and around their back with a snap on it that attached to the carbine via a ring. In short, this was the first single-point sling (and we think we are so high-speed today because we use the same concept!)
Well, the U.S. Army developed its 19th Century guidance by keeping up with the Europeans and by the 1820s, American cavalry also carried short-barreled single shot carbines attached by a sling and saddle ring arrangement. This continued for over 70 years, with the last U.S. military issued saddle ring carbine being the M1896 Krag-Jorgensens that remained in service with National Guard cavalry units until World War One.
As you can see in this picture, with the U.S. Cavalry trooper to the left with his 1873 Springfield carbine and the Union horse soldier to the right with his Hall Model 1836 breech loading percussion carbine, both are hooked up to standard cavalry slings across the user's chest-- through the saddle ring. (Library of Congress images)
Marlin's saddle rings
In the 1860s, popular lever-action rifle makers such as Sharps, Spencer, Henry and Winchester produced models of their shorter barreled carbines complete with saddle rings just in case the Army or a local militia unit (before 1903 each state and county was responsible for arming their own) was looking for guns. When John Marlin's first lever-action repeating rifle, the Model 1881 took shape, it didn't have one, but when the 1889 came out, just to keep the bases covered, JM made sure short-barreled models had a ring.
MARLIN MODEL 1889 LEVER ACTION SADDLE RING CARBINE. SN 86394. Cal. 44 WCF (44-40). Rare Model 1889 carbine with 20" rnd bbl via Julia Auctions
Off and on over the next 70 years or so, various model 1889s, 1893s, 1894s and Model 336s-- primarily very short-barreled models left the factory with saddle ring options. They weren't very common-after about 1900 basically just marketed for nostalgia reasons-but they were made.
Rare Marlin Model 1893 Saddle Ring Trapper Carbine with 16 Inch Barrel via Rock Island Auctions
Marlin Firearms 1893 30-30 caliber rifle. Excellent saddle ring carbine via Collectors Arms
Marlin 94 .44-40 caliber rifle manufactured in 1922. Note the leather tie. These are commonly seen on saddle rings as they help quiet the 'rattle' of the ring. via Collectors Arms
Marlin Firearms 336 .30-30 caliber rifle. Early model 20" carbine with straight stock and saddle ring via Collector's Arms. These brass saddle rings on 1950s era 336s are in fact, factory and not aftermarket.
In 1984, Marlin added a cross-bolt safety as a standard feature to all of their lever-action rifles, which eliminated the option for a saddle ring bar for good.
However, Beartooth Mercantile in Cody, Wyoming makes a $30 steel piece that replaces the factory safety (at your own risk!) on late model Marlin 1894, 1895, or 336 lever action rifles-- with a saddle ring.
Just be sure to tie a little piece of leather or paracord on it to keep in from rattling and beating up your receiver through lots of use.
While the odds of you using it as the old horse troopers did, it is still pretty neat.