This stamp comes in two types I and II, yours is a type II (this can been identified by the shape of the 5 in "25"). Type I where printed in sheets of 100, type II in sheets of 200. The straddle pane discussion does not apply in this instance, as the sheets were not divided in this way. The number on the reverse of the stamp is the sheet number, (nearly all Spanish definitive stamps from 1901-1932 have these numeric, or alpha-numeric numbers on reverse). Off-centre examples are more common than well centred examples, unless they are extreme, (ie with half an impression of one stamp and half of another).