The Model 15-3 went out of production in 1977. The three-alpha serial numbers started in 1980, but normally were not used on the K target guns until sometime late in 1983. Moreover, the ADxxxxx numbers don't show up until 1983, and ADL is not listed in the only release of the three-alpha numbers we have. ADM does show in 1984.
If the combination you are reporting is accurate, this would be a very late example of the Model 15-3 Combat Masterpiece. Might be worth a letter.
Does this gun have a pinned/tapered or unpinned heavy wide rib barrel, and can you post a photo of the model stamping? It is possible it is a model 15-5, or perhaps it was somehow mis-stamped with the 3. Photos would help here.