I posted a scan that showed Kisame can make his exterior look like Samehada's durable exterior, so putting the two together should prove that he has the durability.I just thought of the possibility you being correct previously but not entirely sure. While the exterior of Samehada is his spikes, some parts of Samehada without that exterior can release those spikes
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Those parts don't have that exterior but spikes could be released from there. That probably means Kisame releasing spikes from his body isn't really the exact reason for one to believe he shares the same durability because it could merely be an ability granted to him. My thoughts but who knows :lol
That bottom is also probably just the same as the top but covered in a more durable material allowing it to be the hilt.