As armorer951 said it is an indication that the gun is cocked. It is also an indication of the years of manufacture, as that feature was present from the 41's inception in 1957 until 1978, when the indicator was discontinued.
Since people generally like earlier guns, many feel yours with the cocking indicator is more desirable. Myself, I go on a gun-by-gun basis, but I'm no collector.
Edited to add - I'm one slowwwwww typist.