Go to
http://7.62x54r.net for an exhaustive breakdown of Mosin-Nagant markings.
Here is a brief breakdown of some of the obvious ones.
First, the marked part you photographed is actually the
barrel, not the receiver. M-N receivers are normally only marked on the underside of the tang, under the big screw. Contrary to modern Western practice, the Soviets (and the Finns) serial-numbered the barrels, but not the receivers.
The upper hammer-and-sickle symbol obviously denotes that the rifle is the property of the Soviet Union. The arrow inside a triangle shows that the rifle was made at the Izhevsk, a city west of the Ural Mountains that was home to the Soviets' primary arms-making facility during the Great Patriotic War. Although it's not visible in the photo, the barrels of recently-imported rifles are usually marked with a square symbol with a slash across the middle; this is a postwar arsenal refurbishment mark.
The "r" character after the year is an abbreviation for the Russian word "god", pronounced more like "goad", which means "year". In the Russian language, the numerical year is an adjective, so it's not grammatically correct to say the number by itself without the word; for instance, a Russian would always say "the rifle was made in the 1943 year".
The characters under the year are the original Soviet serial number. FWIW modern importers have to apply their own serial number elsewhere on the rifle for several reasons. First, IIRC the BATFE has decided, in their infinite wisdom, that serial numbers may not contain characters from foreign alphabets. Second, the Soviets adopted a confounding, non-consecutive serial numbering scheme so foreign intelligence agents couldn't accurately discern how many rifles the Red Army had by examining serial numbers. Western sources still haven't figured out how to decipher their system(!) and apparently the BATFE requires that serial numbering systems have to be documented and make sense. Third, possibly as a consequence of the deliberately confusing numbering system, a few duplicate numbers have been documented (see above link), and rifles produced in different time periods at different arsenals have been found with very similar serial numbers.