Thanks in advance for the depth of experience and the generous spirit of sharing I have found here. I have only just dipped my green horn into the world of single-shot rifles, and every question I've naively asked here, has been received and responded to. 

I just traded into an early low-wall x 22 WCF, (don't even have it in hand yet..).

What is the first digit in this serial #? One? or Two?

, and any ideas or concerns about these marks (maybe just from a sharp cut nail over the fireplace?)

can't be hammer dings with the crescents oriented that way. at least, non of my hammers matches those. perhaps it was the result of a long time riding in some sort of place where the roads were really bumpy?
looks like it might be a pretty neat rifle. I had one of that caliber and it shot pretty well with jacketed loads. never had any .22 caliber cast to work with. enjoy it.

1888 manufacture date!

Jeff, you'll find the bore to be .226-.227 groove diameter. My luck with .22WCF has been with .228-.229 diameter bullets (coincidental with shooting light cast loads in .22 Savage Hi Powers). Best luck with .224-.225 bullets was with very soft plain base ones and light charges of Bullseye or Red Dot - giving them a good swift spanking to bump them up to fill the grooves (but not so much oomph as to induce leading - a fine line there).

I use Hornet dies, with a separate neck expander to create a neck diameter that is simpatico with the cast bullets. You only need .001-.002" neck tension not the death grip imparted by the Hornet die intended for .223-.224 jacketed stuff, plus the tight neck may well distort the soft lead bullet diameter you'll have struggled to make to match your throat/bore.