n/a means normally aspirated (i.e. no turbo)
otherwise known as normally asthmatic.
the 2.5 n/a engine (12J) was modified to the 2.5td (19j) by the addition of a turbo (but no intercooler) and therefore some parts are common between these engines.
the 200 tdi / 300tdi (tubo diesel intercooler) engines are very different to the 12j and 19j however.