The alphabet used on ancient Egyptian and Palestinian coins is Greek. Greek has no letter shaped like our letter "L".
It follows therefore that this symbol is not an actual letter; it's there to denote "these letters are actually a year". Just like the lines above-and-below that people often use when writing Roman numerals, or the use of the keraia symbol in
modern Greek numerals, make it obvious at a glance that the letters are to be interpreted as numbers, rather than letters.
Scholars debate the exact origin and derivation of the "L-for-year-symbol", with the dominant theory holding it to be derived from the word for "year" when written in Egyptian Demotic script. The use of this symbol to denote years is, as I said, restricted to Egypt and neighbouring Palestine. It was never used in the rest of the Greek-speaking world, such as on the coins of Tyre, Kyrene or other coinage series that used Greek numerals to declare dates. I don't even think it was used on Egyptian-controlled Cyprus. So an Egyptian-derived origin makes the most sense to me.
Another theory suggests it is a devolved Greek letter "E", since the Greek word for "year" is
etous. But if this were the origin, one would surely expect many other dated coins from other places, far from Egypt, would also use this shorthand.
All of which is a long, roundabout way of saying, "Nobody knows". Sorry.

Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis