UNITA was an insurgent group that fought against the Portuguese during Angola’s independence struggle on the same side as their future opponents, the MPLA. After the Portuguese withdrew in 1975, they aligned themselves with the west and fought against the Soviet and Cuban-backed MPLA. UNITA’s leader was Jonas Savimbi. UNITA received support from Apartheid South Africa and the United States in the 1980’s (source: http://news.bbc.co.uk/2/hi/africa/264094.stm), with Savimbi even meeting with President Reagan in the White House in 1986 (source: https://www.reaganlibrary.gov/archives/speech/white-house-statement-presidents-meeting-jonas-savimbi-angola).
UNITA also recieved aid from China briefly from 1966 to October 1975 (source: https://books.google.com/books?id=LdzJAAAAQBAJ&pg=PA33, https://academicworks.cuny.edu/cgi/viewcontent.cgi?article=1267&context=cc_etds_theses). UNITA fought throughout the Angolan War until February 2002 when Savimbi was killed in an ambush, after which UNITA demobilized its military wing and became a political party (source: https://www.britannica.com/topic/UNITA).
Sample 1:
Olive green HBT material.
Pockets:
Hand-written number on left pocket flap inside, as worn.
Cuffs:
Button:
Inside:
Usage Photos:
Some versions of this shirt have pleated pockets. Original caption in Swahili says “Akisalimiana with his assistants” (from https://globalpublishers.co.tz/kosa-kubwa-alilofanya-jonas-savimbi-hadi-kuuawa/). 
South African soldiers and UNITA in the Angolan bush. Olifant tanks in the back (from https://www.reddit.com/r/MilitaryPorn/comments/9v2zso/south_african_soldiers_and_unita_in_the_angolan/). 
April 29, 1986 : UNITA troops walk through a field, twenty miles from the front line, at Munhango, Angola. The UNITA forces, led by Jonas Savimbi, are engaged in a guerilla war against the Angola government (REUTERS/Wendy Schwegmann – RTR1D6TP; from https://www.voanews.com/africa/angola-rebury-national-icon-and-rebel-leader-savimbi).
Most pictures of UNITA guerillas show them wearing solid green uniforms.