What is this part ? on 1997 Mercury Villager

There is a small uneven hole in a cap on a part attached to the side of the compressor.Maybe it's a relief valve? We charged the ac system and it worked for a day or so. Can anybody tell me what this is?

by in Lancaster, OH on June 21, 2012
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1 answer
ANSWER by , June 27, 2012
MY question would be....WHO charged the a/c system