Hopefully, I haven't offended too many people.
Anyway, here goes .....

I've got evidence that I have "Heatsink" on-board. Left side in my cockpit. Also shows "1/2"
I'm not sure what this means .......

Is it just the number of "items to drop" ?
Does it include the delivery module ?
And how do you deliver them ?
Thanks.
Any weapon, heat sink launcher, limpet controller or something else than fires or launches something into space based on ammunition or items of limited supply such as limpets, will show this:
Ammo in the firing chamber / ammo in reserve that will reload when the firing chamber is empty.
So for the heatsink, 1/2 means there is one heat sink ready to fire and two more heatsinks in the ammo store. When you launch a heatsink, you will see 0/2 until the launcher reloads and then you will see 1/1.
Go into your control bindings and set a key for heatsink launcher. I use 'v'. As an explorer, if you need to charge the FSD very close to a star, perhaps because you ran into the star and supercruise did an emergency stop, launch a heatsink a couple of seconds before you start to charge the FSD and it stops the ship from overheating.
A multi-cannon (Gatling gun type weapon) which has ammo will show something like 77 / 3660.
When the 77 rounds have been fired, the weapon will spend a few seconds reloading and be unable to fire (unless you had engineering done on it for the autoloader special effect, but that's another story). When your 3660 (or however many rounds it is) are all fired, it's time to leave the battle!