My understanding is that 1600 relief valve is better one. I've had both engines when I drove Minx models.
Again IIRC when 1725 was launched in Sept 1965, there were oil pressure issue complaints from owners particularly at hot engine tickover and Rootes bulletin to dealers was to fit 1600 one instead . That it seems gave better pressure on gauge readings both at tickover and at say 50mph. The Series 6 I ran supposedly had this conversion according to original motor engineer owner and still had good pressure at over 100K miles.