RC=Regular Carbine
Do you mean the Remington model 700 bolt action rifle?Marlin never made a model 700 to the best of my knowledge.
Regular Carbine.
Glenfields are Marlins marked for sale by 'second-tier' retailers.
This may- I mean MAY be your rifle. It's a model Marlin Model 49 slide action Shotgun. Exposed hammer takedown combined features of the model 42 and 24. It was never in the Marlin Catalogue. It was made between 1925 - 1928.
10Your glenfield marlin 30-30 model 30 was made in 1966 according to marlin arms chart.
Do you mean ejecting?
The letter J was used by Marlin to indicate what year a rifle was produced.This system was used from 1948-1968.The letter J indicates that your Marlin model 336 SC(sporting carbine) was made in the year 1952.I hope that you find this informative.
This means established 1820.
The letter G prefix to your serial number indicates that your Marlin rifle was made in the year 1950.
Long and Long Rifle.
By "tube" I am assuming you mean the outer magazine tube. If parts are not available from Marlin, would suggest you check the website for Numrich, under gunpartscorp.
First lets start with the 336RC.This stands for the marlin model number(336),The letters RC stand for the words(Regular Carbine).The numbers 30-30 stand for the caliber of your marlin rifle,30caliber,and the second stood for 30grains of black powder.This was how the original load back in 1894 was measured.The F prefix to your marlin model 336 regular carbine indicates that it was made in the year 1949.I hope this helps.