Hello again...
Bruce, the external marking you have post is an example of the old system of registration in which there was not a year reference, only 'list number' (3a, third), 'province' and 'district' (VI-7) and 'page' or 'sheet' (3553). I'm not sure about the correct name of this last field in english... is 'folio' in spanish.
In your example, '3a' means this is a ship of the third list (fishing vessels), 'VI-7' indicates that is registered in the maritime province of Vigo and in the district of Bayona. The rest of the digits is the 'sheet'.
If I'm not wrong and my memory serves me right, I think this method of registration is prior to 1989. From that moment, there is a field for the year (two digits) in the registration number (99 for 1999, 00 for 2000...).