This information migh help you!
According to Browning web site, the BLR was made in Belgium from 1969 to 1972 and in Japan from 1973 to present. K was the product code for the BLR. As yours is made in Japan, the 57 certainly means 75. For a reason I ignore, they sometime reverse the two digit representing the year of manufacture. So it would seem that yours would be manufactured in 1975 if that makes sense.