“This is a fairly rare pedal. The “L” series were never reissued. These Master or L Series were the bridge between the “9” and “10” Series and were only made for one year — 1985 — though some were made in late 84. This particular model has the famed JRC4558D chip inside and is like the TS-10, but with more gain as well as tonal options via the Metalix control.”
“This is likely to be THE most commonly overlooked pedal when listing the various models of Tube Screamers made over time. While the Super Tube STL will be listed among these sleepers it is a much different configuration than the Metal Screamer.”
More information on this series can be found here.