Thanks for the follow up. I suspect you're right regarding the difficulty estimating the date. What I do know is the workshop was founded in 1946, and according to Mr. Takeshi Aoki of Tokyo A-frames, he spoke with Iwasaki and was told that the first tamahagane western razors were made in 1952. It has also been said that the last of the Tamahagane Western razors was made in the mid 1980's. According to Takeshi, the numbers began in the 500's, and the highest serial number that I have seen was 1901.6. The serial numbers correspond to the batch number. Each batch had 9 or fewer razors. Half of the razors failed to meet Iwasaki's stringent inspections. The number after the period corresponds to the inspection number (so 1901.6 is batch 1901, razor #6). So given that numbering started at 500 and ended around 1900, yours is close to the middle of the production. Assuming production numbers were constant annually, which is a big assumption, that would place the production of your razor sometime in the late 60's to mid 1970's.
Or put more simply, there is no way to know, but your razor is probably 40-50 years old just like you said. At any rate, it's absolutely exceptional example. Congrats on a beautiful razor!