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I don't have a specific answer to your question, but I've rebuilt more than a few mec 600's of various ages, and found it to be somewhat hit or miss as to whether you could swap a die between two presses, it usually works though, but sometimes it can be fussy. Most of the rebuilding I did was using donor machines, not buying new parts.
Here are parts lists for both the Mk 5 and "Pre-82" 600 jr's. It appears that some parts carry the same numbers. By comparing the parts you need you might be able to determine compatibility.
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