My Columbia serial number list ends in 1972, but all of the Columbia numbers begin with a letter. I zoomed in on your pic and there's no letter before the 7. They also only have 6 numbers after the letter, for sequence probably. So yours is the 60,539th bike made that year. The 70 may be year, or month and year or even something else.
There is no BMA6 decal on the seat tube. Bikes were required to have that in 71 and later. Your chain guard doesn't match any I've seen from the 70's, so I looked at some 60's Columbia bikes and didn't find that guard or graphic.
My guess is late 60's or 1970.