Hi Emma,
I can't give you the exact dates of manufacture, but the 1346a was launched in the mid-1970's as part of the core Hoover range and was definitely one of the most popular vacuum cleaners ever in the UK. It replaced the earlier style 1346 (which in turn replaced the 1334 style), but aside from the colour scheme, I'm not sure what the differences were. There was a whole blue Hoover range all out in the mid 70's, from the dustette right up to the TOL convertible. Some sources have 70-75 as the years of production, but I have a brochure dated March 1974 which still features the brown 1346 that came before it.