I now don't think it was a MOSFET. Others have matched the JJM JCT425i with a JCT625 and TYN625 (just 600V 25A vs 400V 25a) SCR Thyristor, not a MOSFET.

Does the MD 1618DLC board use these SCR Thyristors and does that cross-reference look right?

 

MD 1618DLC board is Mosfet control AFAIK?

 

It's labeled Q1 on the board - and here is a photo of the board

 

Many of this model used a Bi-polar transistor, IR G4PSC7.
Your version appears to be the N channel Mosfet though.