Steve,
Blueprints created by British Industry at the time will have been governed by British Standard 308, Engineering Drawing Practice. Now I only have a copy from 1953 which I cannot say with absolute certainty would give the same instruction as the wartime edition - but I would be surprised in this case if it didn’t.
Clause 13 a. (v) states:
Where feet are used as one unit of a dimension, it is preferable to express parts of an inch in fractions, thus: 5’-7½" and not 5’-7.5"
And Clause 13 a. (vi) states:
Where decimals are used, any figures preceding the decimal point should preferably be given in inches and not in feet and inches, thus: 25.42" and not 2’-1.42"
Thus to dimension in feet and decimal inches would seem to be in contravention of the British Standard, unless you can show that the wartime standard was materially different.
Rhino supports the two schemes approved by the standard. It does not seem unreasonable that it doesn’t include a scheme that contravenes it.
Regards
Jeremy