I believe it's for Boston Safe Deposit & Trust Co.
added..**** I type too Sloooow!,,, JH1951 beat me to it !! ****
The name as that has been used a number of times and there still is a small bank with that name but fairly new to the scene.
I think the one on the OP's revolver was probably more likely the West Coast based bank used by the railroads from the very late 1800's to just before WW2.
Here;s one link to a BSD&Tr in Ca early on.
#1214 - Boston Safe Deposit & Trust Co and California Southern RR Co - State Archives - Kansas Historical Society
Another one I found in a 1883 yearly report for the Atchison, Topeka, and Santa Fe R/R lists an entry ...
"Totals
Less Boston Safe Deposit & Trust,. deposited with them as Colateral,...."
That site won't let me cut and paste. It has a few references to the Bank in the report in it.,,if you really want to read through it!
Annual Report of the Board of Directors of the Atchison, Topeka, and Santa ... - Atchison, Topeka, and Santa Fe Railroad Company. Board of Directors - Google Books
Well that's my guess aanyway.
FWIW,,the marking looks like a Colt factory roll mark,,not an individually handstamped mark.
Colt would make up a die to do a number of guns on order for an organization if you wanted to pay the extra.
The 'weapon' or rack number was sometimes added aftermarket. Sometimes it was Colt impressed with the same die, the die being made with a replaceable character or characters insert so as to be able to change the #.
It was a common thing on roll dies to be able to mark the same bbl address on rifle bbls for example and be able to change the caliber marking insert.