David, I've posted a pic 20 odd hilts. 
 
Some have the full bulbous base (bungkul), some do not, these "do not" ones have that base similar to yours. 
 
I don't know why, but my feeling is that it is stylistic variation, possibly dictated by available material, possibly because of a need to match proportion to a wrongko. 
 
Or it could be because of damage to the base, but I don't think so. There are too many of these truncated base hilts floating around. 
 
Yes, a lot of the truncated base ones have a fairly indistinct bungkul & it has been carved, also I have a few smaller ones that are on keris that do not have the bungkul. 
 
I lean to stylistic variation.
		 
		
		
		
		
		
		
		
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