Mystery Masakichi Suzuki violin

[TWATANABE]

Hello, I have a mystery violin that I've been scratching my head about for a few weeks. It is a Masakichi Suzuki violin that I inherited from my grandmother, with a 16 coin label and "Nippon" as country of origin, placing it between approximately 1915 and 1921. The mystery is that in the place of a stamped number denoting it's model or violin type (#1-10, S1-6 etc.), it has a handwritten number "1728" where the stamped number usually is. After looking through Japanese auction sites I found a few other examples:

1868

https://auctions.yahoo.co.jp/jp/auction/g1188428243

1890

https://auctions.yahoo.co.jp/jp/auction/s1203965796

1056

https://auctions.yahoo.co.jp/jp/auction/x

1189952026

Of the three, "1868" and "1890" look almost identical to my own. 

Does anyone have any information about Masakichi Suzuki's with handwritten numbers from the "Nippon" era, or a plausible reason why these examples have handwritten numbers instead of stamped model numbers?