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What's the difference between 500-1 & 500-3 TBI?

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  • What's the difference between 500-1 & 500-3 TBI?

    I've been searching for a Holley TBI unit for my 2.5L Iron Duke. I believe the part number is a 500-1, model 3739. I recently found a 500-3, model 3739 listed for a 1.8L GM engine.
    The two units appear identical. They look like the same mounting, both are 1BBL, the air horn looks the same, both are rated at 300 cfm, and both have a 2" throttle.

    Can anyone tell me what the difference might be between the 500-3 and the 500-1? Can the 500-3 be made to work as a 500-1?
    I can't find any information on the 500-3. S-10 forums and Fiero forums often discuss the model 3739, but only occasionally mention the 500-1 (never the 500-3).
    Is there a source of legacy Holley information?