From 1868 to 1885 Stockholms Ångslups Aktiebolag issued a series of tokens depicting a steamboat above two clouds with lightning bolts. Between the latter is in most cases space for a stamped value (one token in the last series, 10 öre, has a raised struck denomination). There are three series:
1. The steamboat has a long vimpel (the elongated flag-like thing) in the bow, there is no dog in the bow and the lowest lightning flashes extend below the cloud at an angle of about 45 degrees. The other side is either blank or shows evidence of previous use as something else.
2. The vimpel is shorter and wavier and there is a little dog next to it. The lowest lightning bolts are longer and extend much further below the clouds. The other side is blank.
3. The dog has disappeared again. The reverse carries Sporrong's older trademark stamp (the oval one).
I have the example below with no value at all and can't find any mention of such a piece in Smith, Stockholms Polletter or anywhere else. Unfortunately the corrosion means that I can't tell whether the dog is present or not, but the angle of the lightning and the shape of the vimpel imply that this is type 2. It isn't 3, since there is nothing on the other side.
Mine is bronze, and according to Smith the only round bronze issues in series 1 or 2 are 10 öre (820GD and 820GL respectively). I can only presume it was intended to be stamped with a 10 but escaped the process.
Your assumption sounds good to me, on the condition that there is a type with an incuse 10. It doesn't look like a filthy die problem. However, it is safe to assume that Sporrong's quality control was not as strict as that of an official mint.
I found this online stamped 8 öre, first type with the dog.
Maybe they just missed stamping your one.
If my unstamped one had been brass like the one Malcolm has posted I'd have had greater difficulty in narrowing it down to likely denomination, since most of the series were struck in brass. Only the 10 öre was struck in bronze out of the round ones.
I think your suggestion is highly likely. What is slightly surprising is that the variety/error is not listed anywhere, as similar cases of missing digits are listed in other contexts in both Smith and SP.
One of the reasons why collecting tokens is such fun is that it is not unusual to find new varieties, even types regularly. You can dispense with marking in a catalogue what you have. Your collection is the catalogue.