The 778 you have is a Marlin/Glenfield version of the Marlin Model 120. It's the same exact shotgun, except the stock. The Marlin 120 has a walnut stock where the 778 has a lower grade wood stock that was distributed to deptartment stores to sell.
It's a great shotgun. I've had both the 778 and the 120 models. I sold the 778 with a 40" barrel that I had. Someone wanted it for the barrel and was willing to buy the whole gun for the barrel. I didn't mind since I had the model 120 with 3 barrels, one being the rifled sight slug barrel.
I paid around $200 for the 778 and a little more for the Mod. 120. I think around $250 with only one barrel. I found the other barrels later on.
The Marlin pump shotgun has quite a few similarities to the Mod. 12 Winchester and it hasn't failed me yet.