Modified MK 18 MOD 0s - MOD 1s?
Upper receivers are not a serialized item, and therefore do not require an "M" or "MK"-type nomenclature - so all of the names/terms used by enthusiasts to describe the "shorty" M4A1 configuration is a fabrication and shorthand. "Officially" an M4A1 is an M4A1 is an M4A1 whether it has a 14.5" barreled upper, a 10.3" barreled upper, or uses components from the SOPMOD program or not.
The supply nomenclature for NSN 1005-01-498-1913 is "Receiver, Automatic Gun," and slightly more generically and descriptively - "Navy issue M4A1 CQBR upper receiver with 10 inch barrel."
"SOPMOD" is the acronym for the Navy's SOCOM accessory development program. The "Blocks" or "Increments" are simply "generations" of accessories, while "Phased Replacement" simply refers to the phased replacement of one accessory within a classification with another.
"CQBR," like "CNVD," "ATPIAL," "ECOS," "4x DOS" etc. - is simply a descriptive acronym that describes what an item does/is.
~Augee