Maybe the Constitution-class is an improvement of the possibly older Sombra-class. The designs may be identical in appearance, but they could have different capabilities and even mission profiles. Constitution-class ships could be considered multipurpose vessels and Sombra-class ships aren't.
		
		
	 
I'm not so sure if the Sombra-class is older now.
		
 
		
	 
That's why I specifically said "
possibly older."  The only thing we know about the Sombra-class is that it's been around the fleet long enough for M'Benga to have served on one before and that the 
Peregrine has a lower hull registry number than the 
Enterprise. Now, that doesn't mean that the 
Peregrine is necessarily older than the 
Enterprise because hull registries apparently are more randomly assigned than sequentially, but it's all we got to work with for the moment. For all we know, the Sombra-class could be variant of the Constitution-class (or vice-versa).
	
	
		
		
			If she was built with the same parts as a Constitution then they were peers?  Or at the time were peers and one class received newer parts?
		
		
	 
M'Benga's line of "Built with the same parts as a Constitution" could just his emphasizing how similar the designs are. A century from now, Doctor Crusher could say the same thing about a Nebula-class ship being built with the same parts as a Galaxy-class. The 
main thing to take away is that the Constitution- and Sombra-classes share a design connection or belong to a design family. 
	
	
		
		
			La'an: A Sombra-class ship, that's unique.
		
		
	 
La'an referred to the design as "unique." That might mean that the design could be fairly rare within the Starfleet.