interquel

As a letter s

is the letter s with a.

As a noun interquel is

(narratology) an installment in a series of works set chronologically between two of its predecessors.

In narratology terms the difference between interquel and midquel

is that interquel is any middle sequel with an earlier and a later installment. This usage may not be proper while midquel is a work that is set within the timeframe of a previous work, rather than before or after it.