You have it. Why don't you look through it? -1 stop doesn't look at all like -8 stops.
Hmm. Good question. It's the only neutral density filter I have. I've never dealt with them before and so have no basis from which to judge.
I plan to be using iso 400 film in an old folder with a fastest shutter of 1/250. In the past a red filter has worked with B&W to get the exposure down to what the shutter can handle. I found this in the drawer and thought it would work for color film.
From looking at this link, posted by Shootar401, I suspect Ian is right. On page 17 there is a chart with densities, transmission percent and fraction, as well as multiplying factor. Factor 2.5 is listed but the densities go only to 2.0, which is 1% transmission. Factor 2.5 would make this one a ND 0.4.
Now that I have seen the filter in good light, I feel really stupid for even starting this thread. It says very clearly on the edge of the filter - 2.5x multiplication factor - meaning a little over 1 stop. Thanks to all of you for your help.