I showed an 1881 some time back. Here is one on a 1900.
Excerpt form my August 2009 World Coin News column:
In the last installment of this column one of the items I highlighted was neither an error or variety but one that I found interesting anyway. It was a Newfoundland 1881 50 cent piece that had the initials "NO" counter-stamped below the bust of Queen Victoria. I asked the question: "does anybody know what the "NO" stands for?"
In fact, I received three responses so far. The first came in from J. C. Levisque who said: "Though I don't have any info on the Newfoundland half you featured recently with the "NO" logotype counter-stamp, I thought I'd share with you another date that I've had for years. Probably a merchant or individual in Newfoundland counter-stamped these, but for what reason? Maybe he was curious to see if a spent coin would eventually return to him." Levisque sent images of a Newfoundland half-dollar dated 1900 with the initials NO counter-stamped in approximately the same location as on the 1881 I featured. Both are shown here.
Jay M. Galst, MD said, "The "NO" countermark on Newfoundland silver is an advertising countermark of Nicholas Ohman, a jeweler-optician in St. John, Newfoundland. This was probably his hallmark. It is described on page 29 of Brunk's Merchant and Privately Countermarked Coins, published in 2003 by World Exonumia Press."
Greg Capps said, "Tonight I was at the local bookstore with my family and I read the article you penned in World Coin News where you were asking if anyone knew the "NO" countermark on a 1881 Newfoundland Half Dollar you pictured. The mark is that of Nicholas Ohman from St. John, Newfoundland and it is a fairly popular counterstamp. Brunk gives a nice write up in his tome on p. 29."
So there you have it, the mystery is solved!