To me, it just seems wrong, time-wise: I presumed they would understand what you wanted for the trade show when I provided them the list that you gave me. This 'would' is 'will' in the past.
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English 1b3I presume they (would have) understood what you wanted for Tradeshow when I provided them the list that you gave me.
'Would have' is being used even though it is not part of a conditional sentence. I think I recall a second use of would have: implied conditionals... Is this what we have here? I'm trying to think what