"accident" in Diamond Jubilee

I just happened to notice in "Speed Building for Colleges, Diamond Jubilee Series", 1966, on page 473 the outline for the word "accident" is given as a-s.  Have I missed something?  That isn't a DJS brief form, and it doesn't appear in the DJS dictionary.  The book doesn't give any explanation that I can find, just lists it in the "vocabulary preview" and then uses it in the reading selection that follows.

It makes sense as a special law/legal brief form, but it seems like there would be some kind of clarification rather than just randomly introducing it that way. 

I don't think I've noticed any other "irregular" brief forms in this book, although I haven't systematically looked for them.

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