Hoping my grandkids will one day appreciate them, so definitely not parting with them if I don't have to. 🙂
After a bit of digging I think I've found the coin I bought from Colin's collection, this BMC 70 James I Lennox Farthing (#991 on the collection website):
Do please let me know if those aren't the same, but I'm fairly certain they are. Unfortunately despite having extensive documentation of all my 2004 and 2005 purchases, I appear to have nothing from 2006.
I'd completely forgotten I had this coin altogether. I guess 1613 (James I - although I'm aware this post-1613) and 1673 are similar enough numbers and in my head at some point I started associating the 1673 farthing with Colin's collection instead of this one. I should probably try to source a tighter capsule for it so it doesn't shake around as much.