first screening; preview
Refers to the initial showing of a film, often for critics, staff, or a limited audience before general release.
The first screening of the new film will be held next week.
I was invited to the preview.
試写会 is a more general term for a preview screening, while 初号試写 specifically refers to the very first screening.
Compound of 初号 (first number/issue) and 試写 (trial screening). The exact historical derivation is uncertain, but the term is conventionally used in the film industry.