The Firearm Blog has a bit long but educational post about how AKS74U was developed and used.
http://www.thefirearmblog.com/blog/2016/02/12/experimentation-use-and-variants-of-the-russian-aks74u/
Edit: More article
http://www.thefirearmblog.com/blog/2016/04/27/krinks-never-aksu-trial-rifles/
From: The Firearm Blog