Is there more than just 2 bockings, if so what do you use them for, and how do you identify them?
The reason Bocking 4 and 14 are usually mentioned and available seems to be that those hybrids are those with the best results.
In addition to what John said let me add, the term "Bocking" refers to a village subdistrict, in Braintree district, Essex in England. The term was applied to comfrey by Lawrence D. Hills who chose it during his research simply because it was the location of his research. He firgured that way Bocking would be universal and used in any language. Where had he said say variety 4, in Spain it would be variedad 4 and in say Italy it would be varietà 4, etc. Where as Bocking would be called Bocking by everyone.
The next thing people ask than is what happened to Bocking 1-3 and 4-13 and are there any higher than 14. The basic answer is that 4 and 14 ended up being the two that grew best and worked best as fodder and fertility yields.