I googled the part number and several links lead me to believe that it may be the same part number used by Homelite on some of their products. Got the the HomeLite website below. Click on "Order Parts" in the red banner across the top, to the right over the "search" box. Then click "Click to Launch Parts Catalog" and then click the gray "Parts" tab then enter the part number and hit search. It should bring up the part number with a list of several machines over the the right that use that filter. Click on one of those and it should bring up a diagram showing the filter. If that looks like it, you should be able to find it locally or at another HomeLite parts dealer's website. The Homelite site says to contact your local dealer to get the filter.

http://www.homelite.com/service_support/order_parts

When I googled that Green Machine model number 2331 what came up makes me think that's the model number for just the trimmer attachment, not the motor head itself.

Ken, Please notice what I found for my old Greem Maciine at the address listed below. loger

http://forums2.gardenweb.com/forums/load/tools/msg0818113810032.html

I just looked at my Green Machine's air filter to discover it's my version of a generic foam filter. I have cut generic foam filters since the days of foam filters (for B&S engines, S25 Poulan saw's engines, the Green Machine's and etc. My Exceptions! I have not replaced a paper filter with foam. I have not ever noticed one negative with this practice. I have only used this practice on my equipment with a grade of foam that can take the environments of the engines (heat, oil/gas vapor etc). The idea came to me when I start seeing too many engines with no filters or very deteriorated filters. What's your opinions and have you cut foam as filters? Be Nice -- But Honest!! Loger

PS. Experimenting Has Been My Life!

That was definetly my answer-Cutting my own filter is exactly what I just did. Took the filter cover down to the local power equipment shop and had them match up the shape of the filter area to a filter they had in stock. There was a filter for a Redmax that fit real good -just had to cut it slightly down to size. Seems to work okay.
Incidently, this is my neighbors trimmer -it wouldnt start. It looked to me like the original filter deteriorated so bad it just crumbled. Thanks all for your replies and help!