@EJG - Thanks mate! I had an idea it was pre-Vat II and that it had to do with local arrangement re feasts etc.... But not the specifics; a thing you supplied most comprehensively. Thanks again mate!

@ACLumsden
The “external solemnity” of any feast means the celebration of the feast without an office, for the good of the faithful, either on the day on which the feast is impeded, or on a Sunday when the feast occurs during the week, or on some other established day.
357. An external solemnity either belongs to a feast by right or is granted by a special indult.
358. An external solemnity belongs by right only to:a) the feast of the Most Sacred Heart of Jesus, on the 3rd Sunday after … [Altro]

I'm not quite sure what is meant by "External Solemnity"......