Phil et al,
Thought I'd add a little more information concerning the John Jowett firm. The earliest mention I've found is in the catalogue for the Great Exhibition in 1851, where it is listed as being located on Arundel Lane. For what it's worth, the firm appears to be the successor to the short-lived firm of Marshall & Jowett, at the same address. The firm continues at the Albion Works, 60 Arundel Lane address until 1879, when it "removes" to 82 Countess Road, still in the Albion Works. The latest directory listing I've found for the firm is 1925, by which time it has become a Limited enterprise. It may well have survived longer, but I don't have specific primary information to trace that.
As a general rule, I think one can trust the quality of any edge tool made by a longer-lived 19th century Sheffield firm. Early in the century, the edge tool companies agreed to set prices and discounts, which meant that they tended to have to rely on their reputation for quality, rather than price, for their long-term survival. Undoubtedly, some short-lived companies went out of business for reasons other than their quality reputation, but few firms could have survived for any length of time without being able to consistently produce tools which people could count on. A surviving tool, today, may have been compromised at some point in its tenure, but if that hasn't happened, it has quite a good chance of being usable.
Don McConnell
Eureka Springs, AR