Welcome to the AGF!
IME older lower-line Gibsons such as your L-48 had factory order numbers (FON) and rack numbers, but no serial numbers until the early-60's. On an archtop guitar these are located inside the back (the flattops I've seen had them on the neck block), and may be seen by looking inside one of the f-holes; FYI many of these had a letter prefix that would help you narrow down the year of production, as follows:
Z-1952
Y-1953
X-1954
W-1955
V-1956
U-1957
T-1958
S-1959
R-1960
Later guitars would have had both a label (inside the upper f-hole) and a stamped serial number on the rear of the peghead -
both of which would be readily visible - so I'm certain yours is a pre-1960 model.
Be advised that the ink used to stamp the FON numbers often fades with time, so you may have to do a little careful searching to find it (a pocket flashlight like the Mini-Mag would be helpful here)...
Hope this helps...
