Always verify the legitimacy of individuals or organizations before sharing personal info or making payments. Do your research. RSA is not responsible for losses or damages caused by scams or fraudulent activity.
Remington Arms Co. introduced their 12-gauge, five-shot, John M. Browning designed, "Remington Autoloading Gun" in 1905. The company name was changed to Remington Arms - Union Metallic Cartridge Co. in 1911. Shortly after that Rem-UMC began calling that gun the Model No. 11. In 1920 the company name changed again to Remington Arms Co., Inc. Circa 1927/8 they changed the safety from the lever inside the trigger guard to a crossbolt behind the trigger. According to the serial number chronology compiled by James Tipton for his article on these guns, yours would be from 1909 -- 63,486 to 82,042. This is from the 1910 catalogue --
Low brass, high brass, makes no difference. Modern American 2 3/4 inch 12-gauge ammo is loaded to SAAMI Specs. Low priced "Wally World" ammo is often loaded with a smaller amount of hotter powder to achieve the desired velocity, but still remain with in the max allowable 11,500 psi. The heaviest available smokeless powder 2 3/4 inch 12-gauge shells available at the time your gun was made were 3 1/2 drams (equiv.) and 1 1/4 ounce of shot.
No one can tell you over the internet whether a gun is safe to fire, and/or with what ammunition. That can only be done by a competent gunsmith with the gun in hand.