3 rib, 5 rib, 6 rib, etc just describes the external gearbox castings.
3 rib gearboxes were fitted to 1600 Type 1 engines and have the following codes, CA, CB, CC, CD (1968-71), CE, CF, CG, CH (1972-75), CU, CV, CW, CX (1976-79).
CB, CF and CW 'boxes had mountain ratios installed, CC, CG and CV had limited slip differentials, CD, CH and CX had mountain ratio and limited slip diffs.
5 rib gearboxes were fitted to 1700 and 1800 Type 4 engines, codes are CK, CL (1700 1972-73), CM and CN (1800 1974-75). CL and CN had limited slip diffs.
6 rib gearboxes were fitted to 2000 Type 4 engines and had CP and CT (1976-79) codes. CT having a limited slip diff.