Dating older Hi Powers is a very hard process that yields more of a SWAG more than an exact date. It often involves everything from serial number, the actual guns build features & engineering changes, the individual inspector’s marks, the coded part inspection dates. Anthony Vanderlinden's "FN Browning Pistols" book is one of the best references for this. He has a pretty complete list in this book.
As to the "1" The shape/orientation of the digit would change each decade; example, the 1 of 1951 is the typical European style 1 that almost looks like an upright 7 while the 1 from 1961 looks like this "1". From the markings IIRC this would be the 3rd qtr 1951 because the bottom square is missing.
FN was terrible about serial numbers again often using the same number over and over again. For example if someone ordered 5000 guns they might start with 1000 and use 1000 to 6000 as serial numbers. Ad this to the fact FN is known to make design/engineering changes to the BHP and then continue to use old new stock parts for years intermittently. So a "1" firing pin retaining plated did not always go into a 1951 or 1961 gun. This is why pictures of the gun, its parts and all proof marks present are necessary to really nail down the date and even then it is a SWAG.
All that said I asked one of the most knowledgeable FN Hi Power guys I know, sub-moa who is a member here, believes from the serial number alone, assuming that it is not a contract gun, the gun was made in 1953. He has 2 BHPs which are accurately dated as 1953 guns which bracket yours. They are serial numbers 52985 and 54762. So that is my SWAG for the date of your gun. I am not an expert by any means. I am repeating info I have found over the years on forums and in books. I am deferring to Sub-MOA's knowledge and extensive research but I have never found him to miss on such things.
If you are a BHP fan I recommend getting the R Blake Stevens and the Anthony Vanderlinden's books. They are not 100% accurate nor complete but they serve as 2 of the best reference points for these wonderful guns.