I'm looking for tiny hex heads that can be placed on a model. Specifically I have two Stuart tanks in my stash which need extra hex heads applied to the holes on the transmission cover edges. I've tried looking for them, but the hard part is describing what these things are to begin with!

You mean hex nuts? Meng Models has several sets. I find them on eBay all the time.

https://www.bing.com/images/search?view=detailV2&id=7ABEB2C6B760E9B0EF8B571DBC871B62DF54677A&thid=OIP.4kadvPFNCPVrGS0JwxspCAHaFj&mediaurl=https%3A%2F%2Ffarm5.staticflickr.com%2F4552%2F37899317294_673cd26b6d_b.jpg&exph=768&expw=1024&q=m5a1+tank+walkaround&selectedindex=27&qpvt=m5a1+tank+walkaround&ajaxhist=0&vt=0&eim=6

Just below the front lip.

You can buy various after-market hex-nut sets, as Evan mentioned, or...

You buy a styrene hex-rod of the reasonable size and cut tiny bits from it to glue on as hex-nuts. IF you cannot buy the size that you want, you can heat and stretch the styrene hex-rod just like stretching sprue from the kit. The stretched rod will keep its hex shape as it stretches out, leaving you the chance to cut tinier "nuts".

Punch and die sets also offer opportunity to make some sizes of hex-nut punched from a sheet of styrene.

The AM hex-nut sets do offer nuts with tiny bolt-nibs in some cases - a level of detail not to be had readily from the above "DIY" routes. But you will pay money for that detail.

I would start by looking at the real photos (to determine whether the bolt ends were actually pretty visible above the nut - some were and others not so much - and also to get a good sense of what the real and later, your scale size, are), and make a decision from there.

The decision would (for me) be: buy a hex-set if the nuts show bolt ends AND they are fairly prominent on my kit, OR, DIY if the nuts are small on the kit and / or don't show obvious or prominent bolt-ends.

Just my opinion!

I usually DIY - done lots of these tiny hex-nuts without even bothering with using hex-rod. In many cases, it's just a tiny nubbin standing proud above a rack-bit or flange or something, and a tiny nubbin without detail will really suffice for all viewers. But I'm notoriously frugal about these add-on bits, so...!

Cheers! Bob

Look like bolt heads to me.

Get yourself one of these tools and make as may as you need.

I’m on the same page as Evan. Maybe this link helps? MENG Nuts and Bolts

Depending on how much modelling you do, and how often you think you might need to make your own hex nuts or rivets, consider one of these. There tools are well designed and well made (as in a lifetime purchase), and they have great customer service.

https://thesmallshop.com/collections/photo-etch-rolling-and-forming-tools/products/sms009-the-nutter-set-model-nuts-and-rivets-tool

Thanks. I need a few, not a vast number.

If you are looking to be frugal and not to buy a new tool, take a spin through the craft store glitter aisle.

Some brands of kids craft glitter are hex punched, some square, and some round.
A cheap container will go a very long way.