Are they to do with production date at all?

While working on my Hornby Thomas, I noticed a number cast onto the chassis block: "7 - 86". This seemed like it might be a date, but I wasn't entirely sure. I decided to compare with my other Hornby engine, Duck, who is much newer - he has a number cast in the same place: "S 4681". That… doesn't seem like a date.
So, what exactly do the numbers mean? Has it changed over time?

I'll share what I know of the age of the models, in case that's relevant.
Duck, I know to be from 2006. Has that in the Gullane copyright tampograph on his keeper plate, and got him as NOS in the 2006 box.
Thomas, I don't know exactly when he's from. I know he's pre-1999, because his keeper plate says "Made in Great Britain", but how old exactly is unsure. "Hornby" is in italics, if that helps.