Code:

l) the Pair of Iron Shod Boots 'Avanc' [4,+18] <+14>
Dropped by a blue yeek at 100 feet (level 2)
+14 speed.
Cannot be harmed by acid, fire.
Slows your metabolism.

Ok what's the chance of getting this at 100ft from a blue yeek??? Let's look at the code...

Base level is 2. First we try to generate a base item of level 2. Iron Shod Boots have a base depth of 5, which means that you must pass the item boost check (1/20 chance). Then you need to generate Iron Shod Boots explicitely out of all possible combinations of items at depth 5. Let's just say all items are Iron Shod Boots and keep the allocation chance of 20 for that item in object.txt (1/5 chance). We're already at 1/100 chance of getting some basic Iron Shod Boots...

We only get one roll at making the object (not a "good" drop). We apply magic, allowing artifacts but not getting extra rolls at it, which means we have to pass both "good" and "great" rolls for the object. At level 2, the chance of getting a good object is 35% and the chance of getting a great object is (fixed at) 30%. That's roughly a 10% chance of getting a roll at creating an artifact. So we're now at 1/1000 chance of getting some artifact Iron Shod Boots...

Now we browse the list of artifacts and find one that matches the tval/sval pair of our base item. It must now pass the different artifact checks: depth and rarity roll. I open the "randart.log" file to check these values for our artifact boots 'Avanc': min depth is 47, alloc prob is 12. So that's a 1/8 chance of passing the rarity roll and a (47-2)*2= 1/90 chance of passing the depth check.

Result: the chance of generating that artifact is 1/720000. Without taking into account the chance of getting Iron Shod Boots from the pool of all level 5 items...

Chance of seeing that happen again? Probably never...