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Do you or anyone else have any idea how the number 3L54 was designated by Scott?
The "L" prefix isn't too common in the Scott catalogue - it's unusual enough that it isn't listed among the more common types in the introduction at the front of the catalogue ("B" = semi-postal, "C" = air mail, "F" = registration, etc.). It does show up here and there, however. It's a sort of a local stamp, "local" in the sense that it might have only been used in a particular area of the country.
The German local officials are fairly common - here's Scott OL4, used in Prussia.

Much less likely to be in your collection are stamps from the cantons in Switzerland, issued before the confederation began issuing stamps in 1847. This is Scott 3L1, used in Basel. (The 1L stamps were from Zurich, the 2L stamps from Geneva.)

Australian Antarctic Territory stamps are given an L prefix, as they are supposedly intended for use from the Antarctic (although virtually all of them you see were used in Australia proper, where they are also valid for postage). Here's Scott L61.

Stamps from the Ross Dependency are listed under New Zealand and are given the local prefix. This is Scott L3.

In the case of your Yugoslavian stamps, the 3L prefix indicates they were used in Slovenia. 1L stamps are from Bosnia & Herzegovina, 2L stamps from Croatia-Slavonia, and 4L stamps are from the Carinthia Plebescite.
Ryan