Now THIS is a fun question! First, this designation refers to a tire, not a wheel. I remember when I was Jared's age, back when Camaros and "Stangs" ruled the earth, I had a friend that had a set of "L-60" series tires on the back of his '67 Camaro 327. Those were BIG FAT TIRES! Definitely aftermarket, and for the drag race set. His tires were almost "slicks", and as I recall, a few miles later they WERE slicks! Not much tread on those bad boys, especially with that SBC (with Holley 650 double pumper and "Three-Quarter" race cam, whatever THAT meant!) spinning them into smoke!
Now for the question at hand. A quick Google search (I just LOVE Google!) and a table popped up from the Tire Rack site (the table, for those that want the compleat story, is at:
http://www.tirerack.com/tires/tiretech/general/size_conversion.htm )
The Alpha-Numeric sizing was roughly equivalent to the older imperial numeric tire sizing, and designated the load carrying capability of a tire, and to some extent, the "Section" width, that being the widest part of the tire. The "L" series tire had a 9.15 to 9.50 inch section width, depending on diameter. The "Series" of a tire (the "aspect ratio", or the ratio of sidewall height to width) determined the diameter of the tire (or how "tall" it was). A standard tire back then was usually a "78" series, so the sidewall was 78% as tall as the section width. Ergo, an "L78-15" tire was pretty tall at 29.3 inches. Now a "60" series tire with otherwise the same designation was only 26 inches tall. I don't think that 50 series or smaller tires were readily available back then. Certainly not the 35 series tires that are on modern high performance sports cars today.
Anyway, I digress. It is possible that your dad did indeed have a set of "L" series tires that could have been nearly ten inches wide. I recall there being quite a few guys with Celicas and Corolla SR-5's that had "Grabber" 60 series tires. I don't know if they were "L" sized, but they could have been.