"SQ" in part number designates Square Flange design with holes drilled in corners.
"RC" in part number designates Square Flange with severely rounded corners (no holes).
"A" in part number machined from aluminum - "B" from brass.
All boots machined from aluminum ("ST" - Straight)("AN" - Angled).
"BOX" fittings are computer mounted with one boot needed.
"LINE" fittings are wire hook-ups with boots at both ends

These 1801 series fittings are "Box Fittings" (as shown on the left of upper photo) in 1/12 scale and are used on computer modules where only one exit boot is required. These are the largest of the two sizes available.