The model started out as the model 27 in 1909, and with the improved safety as the 27S in about 1911. It was made up to WWI, and again after the war until 1932. The calibers were .25-20, .32-20, and .25 Rim Fire. The model 27/27S was around longer than any other Marlin pump action rifle. They are very solid guns, and have a good reputation for quality, although they were never a high dollar item.
These guns were available in either full octagon or round barrels, of 24" length. Special sights, checkering, and engraving were optional on earlier guns, but post WWI era guns tend to have no special options, except sights. Only straight grip models were made.
The 27 was designed by John Marlin and LL Hepburn. All the model 27S' have a serial number. You'll need to pull the buttstock, and the serial number is located on the left side of the upper tang.
Hope this helps. Here is a link with the above info, these fellas sometimes can give you the right answers on extinct model Marlins. Marlin Collectors Association • View topic - model #27