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When that happened to me recently, I smacked the underside of that
plastic 'guard' that's under the radiator and the screw jumped out.
www.slhondaparts has diagrams and part numbers for everything.
www.helminc.com has the service manual you need.

'Curly'

Look at one of the three remaining and go buy another?

Last year I did exactly like you.  I cleaned my filter and dropped the
bolt when putting it back together.  I couldn't find the orig. bolt
either so before finally going to the dealer for the new bolt, I
called my repair shop.  They told me their mechanic did the same thing
the day before and didn't know the size.  Hence I went to the dealer
for the new bolt.  I don't recall the size now and I think I paid
about $2 but I had to travel almost 1 hour round trip so the new bolt
was expensive.   I learned from that experience and later the same
year chose to replace the filter (wish I did that to begin with
instead of cleaning the filter) and decided to let my mechanic do it
instead.  Afterwards I checked for missing bolts and all was fine.
Unless they sell a special tool, I think you need very nimble fingers
for this job.