David,
240 is 24" belt. The odd thing about the number you posted is that all of my books showing timing belts, (Gilmer) lists L as 3/8 pitch and H as 1/2" pitch. The 12 might mean 1/2" width but don't know why it has L and H on the same belt. Go to mcmastercarr.com and look up timing belts, see if this number is what you are needing 6484K149.
Curtis