I don't see anything apparently rare about that example, either. However, a quick check of the Scott Specialized Catalog of US Stamps includes this note at the end of the listing:
Quote:
"The tagging omitted error No. 2170b appeared before No. 2170a was issued. Plate blocks from plates 2 and 3 are the error if untagged. No. 2170a is from plate 4. Other blocks and singles are indistinguishable, unless se-tenant with tagged stamps."
So it may be that you have a tagging omitted error stamp. You would have to examine it under UV light to tell for sure. If it is the error stamp, you have a relatively scarce item that has a $75 catalog value in mint condition for a plate block. Of course, as a plate number single your example would have a lower value, and even if it is that variety, the value is diminished by someone having written on the selvage.
Also keep in mind that this stamp was produced with "dull gum" so just because it may appear that it doesn't have gum, may not necessarily be accurate, as the "dull gum" is often mistaken for "no gum".