The biggest obvious difference on the outside is that the HF Antennae on a Mk.3 goes from the fin to the top of the Port side Sponson, inside which is where the HF transmitter/ATU is located. On the Mk.3A the HF Antennae starts at the fin but is then turned through 90 Deg downwards towards the Hull about halfway between the Port Bubble window and the Port Sponson.