Early Predecessors to the Twin Cams:
EX-172 - 1952 not an MGA, but a rebodied MG TD built to run at La Mans.
EX-175 - 1952 First factory prototype for MGA, based on 1952 EX-172 Le Mans car.
EX-175 - 1954 (2nd car) still with MGA body style, Bonneville speed record 153 mph - Also averaged 120.74 mph for 12 hrs. Combined an MGA chassis with an unsupercharged modified 1500cc TF engine.
EX-179 - 1956 EX-175 from 1954 rebodied as streamliner, now Twin Cam 1600 engine. Bonneville speed record 170 mph. At one time held almost 100 International and World speed records.
EX-181 - Streamliner. 23 Aug 57 - 1489cc Stirling Moss 245.64mph (395.31 km/h).
EX-181 - Streamliner. 3 Oct 59 - 1506cc. Phil Hill 254.91 mph (410.23 km/h), to this day still the fastest ever MG.
EX-182 - 1955 - Three built for Le Mans, redesign from EX-175 with new B-series engine. Competed in Dundrod Tourist Trophy race 1955 with Morris designed Twin Cam 1600 engine, which was eventually used for the production model.

Prototype Twin Cams:
ND?/13/498 - More information
YDL/14/499 - PJB147 - More information
NDL/13/500 - ORX885 - 1958 Twin Cam Prototype. - More information

Special Production Twin Cams (Works Cars):
EX-182/41 - 1958 reincarnation of the 1955 Le Mans race car EX series, this time built with a new twin cam chassis and aluminium body. The chassis number is not known (at this time), so it is hard to say if this is a pre-production or early production Twin Cam works car. - More information
EX-186 - 1958 reincarnation of EX-182, Twin Cam engine, never saw competition. - More information
SRX210 - Modified body MGA Coupe, Twin Cam, Le Mans 1959-1961 - Probably built using an EX-182 aluminum body shell. - More information

Also see: Production Twin Cams and Notable MGA Deluxe