Be sure you tell your supplier to order with your air inlet configuration in mind. A ProM (Mass Air Systems) meter set up for 24's has a slightly different calibration for 1) meter on a stock air box, 2) conical bolted to the end of the meter (don't do that - hot air kills HP), or 3) CAI with an elbow in front of the meter. Anything that effects HOW the air flows across the hot wire in the meter will impact it's accuracy. Also, it's trying to match a transfer function in your computer - so a CAI intake set up for 24's will have a different calibration if it was for an 4) 89 HO, 5) 94 HO or 6) 93 Cobra.
So, if you buy a used meter set up for 24's -- you also need to know was it originally calibrated for 1, 2, 3, 4, 5 or 6 above. They're not as 'plug and play' as folks think...