Fiat changed things between the 1300 and 1500. Or so it appears to me. The 1300s I've had experience with had the special bolt as above - 4152332,
the 1500s just had a stud (cost savings?).
When Fiat changed to the stud they also eliminated the washer - 4325800 - as in the above illustration, because it would serve no purpose. If you do not install the 4325800 washer with the 4152332 special bolt the bolt will bottom out in the blind hole it's screwed into.
If you experience loss of coolant when you remove either the stud or 4152332 bolt it's likely due to the cover plate leaking - not that the stud/bolt hole is open to the water jacket. (They aren't on any of my engines at least.) One thing to note is that the special bolt with the washer actually holds that cover plate down. The stud does not.
A bit off the topic, but relevant, is how to take up slack in the timing belt tension which is introduced when milling the head and or block. Since the 1300 timing belt tensioner bearing is a bit larger in diameter, it can be used to take up some slack on the 1500 engine. But, you've got to come up with some other way to do it on the 1300 that's been modified. I suppose there are many ways to do this, but I've used these two:
First, simply machine up a sleeve to slip over the outside of the tensioner bearing. That increases it's diameter thus taking up the slack. It works great.
Next, you can also modify the pivoting bracket that holds the timing belt tensioner so that it will pivot out just a bit more, taking up the slack. This is a bit trickier, but it's possible. Here's are a couple pics of mods to the bracket and the special shouldered washer that allow the tensioner bearing to pivot out just a bit more. First the unmodified bracket (with 4325800 washer),
Then the modified bracket and stepped 4149974 washer.
The bracket in the pics above has also been modified to use the special bolt by milling out enough material underneath to accommodate the hex head on the special bolt. The washer (4325800) isn't used as it would limit the amount of pivot.
So...the special bolt must be shortened by about 4mm so it doesn't bottom out in the bolt hole. Of course you could use the stud from a 1500, but then you would loose the clamping force that the special stud exerts on the cover plate to help keep it from leaking. (Note that there's not much on that corner holding the plate on.)
A third method that I've seen, but not used, is to elongate the pivot hole on the bracket. I don't recommend this, but It seems like it might work.