The 99M1 is a carbine version of the Mod 99/60 family of Marlin semi-auto's. As you know it features a wood hand guard and is tube fed with a the magazine tube encased in the forend. It has a couple cousins: the 989M2 which is very similar but is clip fed and the Glenfield Mod 75, which is tube fed like the 99M1 but does not have a hand guard. The Glenfield versions were also supplied with a hardwood stock rather than walnut, which is usually found on the 99M1 and 989M2.
Unique to the 99M1 and 989M2 was an adjustable rear sight that looked very similar to the M1 Carbine. The Marlin sight clamped to a rib along the top of the receiver. Many removed the sight to mount a scope. Those sights are becoming rare and costly. The Glenfield, not having a hand guard, received conventional Marlin rimfire sights.
As with the entire Marlin semi-auto family, they were/are very reliable and accurate with ammo that it likes. When properly maintained, very little will go wrong with them that a good cleaning will not remedy. They do look surprisingly very much like a U.S. M-1 carbine.
To the Marlin collector, the M1/M2 versions are much desired, especially in nice condition.
EDIT: 99M1/989M2 Manufactured from 1966-79:
Hope this helps.
From top:
989M2
99M1
Glenfield Mod 75
