IMHO The cursive capital "L" inside the proof mark stands for the Liege Belgium proof house. More photographs of the area above and both sides of the chamber are needed to be sure but just with the photo already provided of the top of the chamber I believe the date has been polished off before this proofmark was applied. There are minute traces of polishing or fine sanding visible below the blue in this area and the receiver corner reliefs appear "softened" instead of sharp.
Just my tuppence ! 
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regards, -John S
"...We hold these truths to be self-evident that ALL men are created EQUAL and are endowed by their Creator with certain UNALIENABLE rights, and among these are life, LIBERTY, and the pursuit of happiness..."
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