This period of First Day Covers in Barbados is quite interesting, as most issues were simply available on plain envelopes, onto which a relevant cancel was added. It was not until 1965 that a regular pre-printed illustrated cover was used and even then, it appears to have been sporadic at times.
Most covers of the time have either Registration Branch, Circulation Branch or Parish cancels, with some of the Parishes being exceedingly hard to find. The first ‘Philatelic’ cancel that I have was 1968 and even then the cancel didn’t say FDC or First Day Cover. Currently, the FDC with the most variations on a printed cover is the 1967 UNESCO issue, where I have managed to obtain three different printed covers. It’s not clear why this issue should have the most different types of printed cover, but it will be fascinating to see, over time, if there are other issues that have more than this.