It very much depends on what you define as aim range, which is not specified there and therefore does not interest me.
BTW I see, the edition is from 1965... I wonder if it is possible, that during about 10 years between first model of B-10 and the edition of the manual there was some modernization of sight. It looks quite different than more primitive equipment of SPG-82. What I suggest is, that 1000 meters were used with early models from middle fifties and 1400 meters applies to newer ones of middle sixties. But that is only hypothesis
Regards,
badger45