does anyone know what this is? splint?

[Splish & Masons mum]

My friend's horse has come in from the field with this on her leg:

It's hard and feels bony, but she's not bothered by you touching it, it doesn't feel hot, and she's completely sound on it.
our initial thought is that it's a splint, but we thought it might be a bit high up? The vet is coming tomorrow, but I thought I'd ask on here to see if anyone has any other bright ideas!
If it does turn out to be a splint, has anyone had any success using Splintex? or would you recommend something else?

 

[NicoleS_007]

Splint, my boy had one in the same place

 

[Phoebe+Sophie]

I think my pony has a splint on her cannon bone too.

 

[Daisy11]

100% splint my gelding developed one exactly in the same place