'Primrose' (iii) was a British exercise to test the practicality of firing self-propelled guns from the decks of tank landing craft during the assault phase of an amphibious landing (March/April 1943).

Undertaken in Kilbride Bay, 'Primrose' (iii) was probably the first time that such an experiment had been trialled. A British secret weapon, the so-called Canal Defence Light, was also trialled in the same exercise. The CDL was based upon the use of a powerful carbon-arc searchlight to dazzle and confuse enemy troops. A demonstration had shown that the use of a vehicle-mounted searchlight both disoriented the units facing it and masked activities behind the searchlight. The searchlight was mounted in an armoured turret fitted to a tank. Initially the Matilda tank was used, its standard turret being replaced by a cylindrical turret accommodating the CDL, whose light emerged from a vertical slit, and a machine gun. The Matilda was later replaced by the M3 Grant which was superior in several ways: it was a larger and roomier vehicle, better able to keep up with tanks such as the M4 Sherman, and it had a hull-mounted 75-mm (2.95-in) gun unaffected by the replacement of its normal turret (carrying a 37-mm gun) with the searchlight turret. The light could be varied in two ways to further enhance any effect: the addition of blue or amber filter to make the light source seem further away or closer respectively, and the operation of a shutter to create a flickering effect. Trials of the CDL in 'Primrose' (iii) indicated that the CDL was 'too uncertain to be depended upon as the main feature of an invasion'.