I see Karl and Phrank have you covered on what the phrase is! Here's a little background on its use.
The earliest verifiable date I can find is from August 9th, 1810, in an ad that ran in the American Citizen newspaper from NYC.
Attachment 306438
Next we have a 'Magnum Bonum' etched razor and strop case illustrated in Smith's Key (1816)
Attachment 306440
Attachment 306441
In practice, these looked a little different, with the etching on top of the spine. From the examples I've seen, it looks like these early 'Magnum Bonum' razors were made from blank patterns purchased from a common source (with some variations).
By the mid 1820's, the etching was on the blade face and there wasn't any particular style associated with the slogan.
Yours dates to between 1830ish and 1850ish, with the middle being most likely.
Enjoy!