"For nominal sizes below 0.5mm, the deviations shall be indicated adjacent to the relevent nominal size(s)"

I would ask the supplier of the drawing what they want applying to the dimension. It's likely they (like me) didnt realise the standard stopped at 0.5mm. Be prepared to suggest a tolerance yourself as it may be possible they have no idea what would be considered reasonable in that range.

Will Walters
Sheffield UK
Designer of machine tools - user of modified screws

Go here:http://www.ams-prototipi.it/webfiles/iso.pdf
Read the note on tol. for under 0.5 mm.

As above go back to the whoever you got the drawing from.

If you can't get an answer and insist on proceeding at risk I'd just do the best you can, and make sure you at least hit the tols for medium .5-3, however I strongly discourage this.