Spokes Thread Size CA78 Dream

[quidger]

I am relacing the rear wheel spokes on a Dream that I am restoring. Three of the spokes have had the tops snipped off on the threaded end, leaving a jagged end on each of the three spokes. Likely snipped off because they stuck out a bit when originally laced.
The nipples will not thread on because the threads have been distorted when the tops were lopped off.
I need to file off the jagged end and re-tap the end of the threads. I'm not sure of the tap size. I tried a 4mm x .7 and it seems to be off just bit when I try to thread the nipple on.
Anyone know what size thread and what size tap that I should use to get the spoke threads cleaned up.
Thanks for any help.