I was at the National Videogame Museum today in Frisco Texas and saw this system looking at the display is this the development NES system?
https://www.ebay.com/itm/American-Automation-AA-547-EZ-PRO-Development-System-/301341145328 I don't know. Sure does look like it however.
What I heard jives with what others are saying here, there wasn't really a specific dev system for the NES.