I have seen this old tool, but have no idea what it is or it's use. Measures 7 1/2" long & 1 3/4" at widest point

Looks like a very early foundry moldmakers tool

This tool may be one of those whose origins lie in the mists of history.  I have several (yours is missing the wood handle) that I've mostly found among old kitchen tools, where it was probably used for coring or cutting strips of things.  You'll also find them among the tool kits of ceramic workers, were they are used for sculpting and hollowing.  (Ceramicists recklessly steal tools from other professions.)  I can see how it would be handy for foundry work too.

I found a clue.  One of these tools showed up on eBay (250941478501) and is marked Ontario Knife Co.  This, I think, is a clue, and I checked for old/antique kitchen utensils/tools, were I found:

eBay # 260901059140 -- a group of kitchen utensils

Then found:

http://pages.pumpkinsandpeacockfeathers.com/image.html?s=http://www.tias.com/stores/pandpf/origpics/ki4a.jpg&p=8.50&t=Antique%20Tiny%20Kitchen%20Scoop&n=Pumpkins%20%26%20Peacock%20Feathers&z=800&b=&l=http://pages.pumpkinsandpeacockfeathers.com/7273/PictPage/3923652194.html

Checking just for melon/vegetable ballers I found the modern equivalent -- oval melon and vegetable baller, aka potato baller.  These are also included in sets of "chef's garnishing tools."

My first thought was ice cream scoop but I wasn't going to say anything... Until I saw all the kitchen references. :)

can you make out what the marking says?

The best I can figure it reads No.5800   Very hard to read

Well, these days you can't use plain language without getting into trouble, but we used to have a name for such tools, and to me that one looks like a really well made, general purpose version of it. The name starts with an f and has 2 syllables, ends in stick :) Or , 'spud ended pry bar' in polite circles. I did not learn the term I am referring to in a polite circle.

Dang Kxxr, thanks for the good laugh, gotta love your analysis. We musta hung in the same circles.

Kxxr, Gotta chuckle, but the business end of this thing is about 7/8 of an inch long, and unless you're working with leprechauns...