I thought the shafts were the same - in fact I am 95% sure I measured both and they were the same.

Many times the ends of the shafts get flared out when the plug gets installed. That would prevent the shaft from going into the pedestal mount. Its usually not an issue with the newer heads but would be with these.

Years ago I bought new DC shafts and they would not fit on my Crane cast rockers because of this issue. I had to grind the ends down to get them on.