After soaking overnight in penetrating oil, you can bang on the receiver with a heavy plastic or hard rubber mallet to try and jar the mechanism loose so that the spring pressure will disengage it.
As an aside, on another forum there was a discussion about a study done by the ASE to evaluate the effectiveness of several types of penetrating oils. The best performing penetrating oil (i.e. freed a rusty nut/bolt with the least torque required) to be a 50/50 mixture of automatic transmission fluid and acetone.
Jim
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