There is one currently for sale on US Ebay, ...
Not any longer - did you (Sanborn22) by any chance buy it?
I don't find this weight in my database - I could just miss it, but there is also a fair chance it is an "undocumented" weight, i.e. not showing up in Caithness sales brochures (would not be the first one!).
So the only clue we have got is the hand engraved code - which I am not sure what it says. It is a code for an unlimited edition (which could have been quite small - just not a predefined limit), and these codes always start with a letter (indicating the year of manufacture), followed by a running number for the n-th unlimited weight made this year (which could be from 1 to 6 digits).
First attempt: I314128 - can be ruled out for two reasons: I was not used (as too easy to confuse with 1) and Caithness never made 314128 unlimited weights in a single year (there were a few years with just over 100000)
Second attempt - assuming the first two characters actually are just one, badly written character - what could it be? Could be a B (for 1984 or 2004) or a D (for 1986 or 2006). As was already pointed out - this is more likely a more recent design - so I would go for either 2004 or 2006 as year of manufacture (design could be somewhat earlier).
If you look through ScotlandsGlass you'll find a Dancing Seahorse of 2002
, engraved by Willie Bain. Although "your" Seahorse is somewhat different, Willie Bain might be the best bet - so 2004 will be more likely.
I had contacted the seller and requested permission to use his images for ScotlandsGlass - no response (yet): is it OK to use your images of this weight?
Best regards - Wolf