The inter-spouse exemption no longer applied for deaths on or after 13 November 1974 re estate duty. Re deaths between 22 March 1972 and 12 March 1975 the threshold was £15,000.
After 12 November 1974 the spouse exemption was unlimited so if the husband died on 18 November 1974 and left everything to his wife and she was UK domiciled, then no Estate Duty should’ve been payable on the transfer. See IHTM43060. If he didn’t use his nil rate band then her personal representatives should be able to claim a 100% enhancement, but I am wondering why Estate Duty was paid?