The serial is custom as the rest of the gun is I believe.
I’ve got a fair sampling in my brace of Bisley models.
Two are typical 5 digit serials in the 27xxx range that flow with the standard single action serials.
One is a custom serial that’s initials and my dad’s birth year .. NC1948
And another that’s a six digit 315xxx
So any guns serial could be about as unique as one wanted it to be if they’re choosing to pay for one, or the otherwise just the standard 5 digit. I’ve had several that have come and gone that were standard 5 digit serials.