That's a beautiful Terrier.
Not all Terrier knives had pattern numbers, not all Robesons, either, for that matter.
Generally speaking, a Robeson pattern number consists of two groups of three digits:
First Number = Handle Material
Second Number = Number of Blades
Third Number + Bolster/Liner Composition
Second Group of Three Digits = The Handle-Die Shape Number
A Terrier knife, however, will have those two groups of digits reversed.
First Three Numbers = Handle-Die Shape Number
Fourth Number _ Handle Material
Fifth Number + Number of Blades
Sixth Number = Bolster/Liner Composition
I do not have a Robeson like that Terrier, so this will require a bit of educated guesswork.
There may be another set of three numbers below the 125.
If that is the case, then the Handle-Die Shape Number for that pattern of knife is 125.
Your Terrier looks to have nickel-silver bolsters and liners. If that is the case, the six digit pattern number for it would might be 125 / 633 or 125 / 636. They used both a 3 and a 6 to denote nickel-silver bolsters and liners. I do not know why.
Tom Kalcevic said that the "6" denoted iron bolsters and liners, but I can prove that incorrect with dozens of examples.
However, if there are no other numbers below the 125 and I realize they might be there, but completely obscured by the bolster, then we have another situation.
Robeson utilized a series of pattern number suffixes to designate a departure from the usual pattern configuration. That might be a different blade option or the addition of a bail not usually used or any other number of things.
These suffix numbers included some fractions, 1/4 or 1/2, sometimes stamped as 1-4 or 1-2 and a number of three digit suffixes, 100, 125, 150, 200 or 250.
I have no idea what all of those mean. I do know, from observation, that 1/2 or 1-2 means a different than usual master blade, i.e. a clip instead of the usual spear. I think Case used a similar system, but I believe Robeson's was exactly opposite from that of Case.
Actually, 125 is the suffix I have seen most often on Robeson knives. I do not think I've ever seen a pattern number suffix on a Terrier. Doesn't mean they didn't do it. No reason why they shouldn'thave, that I know of.
But, if that 125 is the only actual number on the knife, it might just be there to denote a departure from the usual pattern and/or blade configuration, etc.
Fascinating knife in great condition and I am envious.
PM sent, BTW.