Hi Lensman57
It's not even close:
The first picture is of the Regal stock and the MK4 action, the second is the same, you can clearly see the battery protruding below the actual Regal stock.
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The batter won't physically fit, and there is not enough stock in the Regal to even try it. The stock bolts are also bigger and a different thread in the MK4, and there is a whole where the Regals gauge is. Side by side you can see the depth difference in the two stocks, it's out by best part of a couple of inches.
Hope this helps.
James