What is that on the montesas shoulder ?

Additional armour, holding a rifle in your right hand will present your left shoulder to the enemy, so it makes sense to have a little extra armour there.

 

The Tikbalang has this bit too.

 

Huh. That's weird.

If it was strictly on the right shoulder, I'd call it a Maximilian Plate. It's actually a specialized piece for jousting, protects the neck against strikes from the opponent's lance. Not sure about the plate on the Tikbalang, though it certainly looks like a Maximilian Plate that is on the wrong side.

 

its basically that, just on the other side.

Note that both the Tikbalang and the Montessa have their Swords in their right hands, this is to protect the neck on the side that isnt armed

 

I always thought this was a particular thing unique to ASA heavy units CAD models. Like, something unique to them. If GdA is resculpted one day, perhaps he will have it too.
But it is a easy way to know that the model is a Montesa knight, anyway (to those who say that all our HIs look the same).

 

On the model, the modified shoulder pad is on the right; the dossier is a mirror image.

 

I was referring to the Tik shoulder pad, and I also got my sides mixed up.

 

Oooooooooops, that Maximilian Plate should be on the left shoulder, not the right. It's supposed to be shield-side!

 

Then we have our answer. Both the Montesa and the Tik have the plate on the left shoulder (the non-sword arm):

Spoiler

As a side note, as you can see in the first pic the plate came with the last Montesa model. I'm guessing it's mainly here to help differentiate the armour from the Hospitallers and other PanO HI. It also fits the theme of the Montesa being a very aggressive force: their armour is lighter overall (compare the legs for example), but reinforced in critical places.