Not sure that all those variants "count", but to my understating there are six of them to the word clipeus:
Those different variations had raised several speculations in the ancient's minds with respect to the meaning and/or etymology of those different terms - even to extant of self-contraction (...
Your first example is called a contracted perfect or syncopated perfect.
When the perfect stem of a verb ends in a vowel plus V, and the ending starts with a short vowel, the V and the first vowel of the ending can be removed. For example, amāverunt > amārunt "they loved", delēvisse > delēsse "to have removed", ...