Use of 12Y4
According to informations discussed in the Wireles Set No. 19 Group, the valve 12Y4 (corresponding to CV523) was introduced in 1944 and saw use in the Canadian Wireless Set. No. 9 Mk I and Canandian Wireless Set No. 52.
It used a socket B5 (usually found in military sets) and could be replaced by a 12H6 with a matching octal to B5 adaptor.
Hope this helps, kind regards Martin