"Able Crewman"
The encyclopaedia has swath of enlisted ranks that are respected by the invention of expensive prop jewellery/insignia, that Roddenberry and Berman did not give a toss about.
CRM1
CRM2
CRM3
PO1
PO2
CPO
SCPO
MCPO
MID1C
MID
Technically, Midshipmen (1st class or otherwise) are Officer Trainees/Candiates (mostly refered to as Cadet (4th to 1st Class) from TNG onwards) not enlisted, though they are typically placed under the authority of a experienced PO or Chief in the field regardless, particularly in the junior years.
Also, somewhat counter-intuitively, the CRM and PO ratings are numbered in ascending not descending order so the scheme outlined above would actually be CRM3 (aka Recruit (E1)), CRM2 (probably the rank of the "trainees" that Scotty refered to in TWOK, (E2)), CRM1 (aka Able Crewman, (E3/E4, similar to USA Specialist or USAF Senior Airman)), then PO2, PO1 and the Chief ranks.
However, a recent rewatch of ENT and comparasions with the RN and USAF systems to the ranks worn on screen (ENT for all it's flaws was the best about showing an enlisted cadre) suggested a simpler system to me: Non-Rated or junior Deck wear no insignia (refered to as Recruit or Trainee depending on level of training), civilian Able Crewman correspond to Crewman
Third Class (one stripe = Ground Forces Lance Corporal), Crewman Second Class wears two stripes (Leading Seaman in RN terms, Corporal in Ground Forces), Crewman First Class wears three stripes and acts as Shift Commander in Engineering etc, for example CRM1 J. Kelly on NX-01
http://memory-alpha.wikia.com/wiki/J._Kelly, and broadly corresponds to a Ground Forces Sergeant.
The second system while no matching the
terminology of the Fletcher system (perhaps PO is used in place of CRM for Shift/Section Leads) does satisfy the "no rank insignia for below Able Crewman" and also the limited amount we know about the ENT and TNG 'grey patch' systems.