I loaded an older disk image of MP (x86) 1.38.3 to check the issue again, and you're right - after refreshing the update information in the Extension Manager, it shows me the latest version, 1.5.0.12!
This is a well-organized and important collection! I had a problem where the Extensions plugin offered on the MP plugin download page no longer worked with MP 1.38 (x86), but after some searching...