its a Remington model shotgun(semi auto)
500 dollars
Id say a good $200-$300
The 878 was discontinued in 1962
They are date code stamped on the barrel on left side in front of receiver. A barrel location diagram and the date code table (month/year) is available on the Remington Society of America.
I think you mean the model 48, no 40 I've ever heard of, and in typical lack of creativity for models in 1948.
They month and year can be determined by the barrel code, all the details can be found on the Remington Society of America here: http://remingtonsociety.com/rsa/questions/barrelcodes (somewhere between 1948 - 68)
See related links
your Remington 48 is worth $500.00 or more
i just bought 1 a few months ago off of gunbroker.com for 350.00. it was in very good shape.
Check the date stamp on the barrel. The table look and details are on the manufactured date link of the Remington Society of America.
No definitive sn data published.