Hi Mike,
Those chamber length markings appear likely to have been done by a gunsmith after the gun left the factory. Attached are the two styles of factory original markings that I have seen with the oval shaped one on Hunter-made guns (pre-1945) and the single-line version on Marlin-made guns.
The first is a 20ga Specialty from 1929, the second is a 12ga Premier Skeet from 1950. The 2 3/4" markings seem only to have been used when that was NOT the standard length on the Hunter-made guns. (e.g. standard 20ga chambers at the time were 2 1/2" so were only marked if special ordered as 2 3/4") Marlin-made guns were inconsistent, as far as I can tell. By then 2 3/4" chambers were standard in all gauges, 12, 16, 20, but were sometimes explicitly marked and sometimes not.
Assuming that the chambers on your gun were lengthened (probably along with the forcing cones), I would strongly encourage you to have a gunsmith with the proper tools measure the barrel wall thicknesses to make sure they are still safe to shoot.