On pg. 354 E in discussing the adverbial forms in -ā like inter-eā ‘meanwhile’ I suggest that these forms are most probably old feminine instrumental ablatives, but I failed to mention another attractive possibility proposed by J-L García Ramón that these forms are old feminine instrumentals < *-eh2(e)h1. If that is correct then these would constitute an addition case of the retention of final -ā in Latin.
García Ramón, José Luis, 1997. Adverbios de dirección e instrumental indoeuropeo. In E. Crespo and J. L.García Ramón eds., Berthold Delbrück y la sintaxis indoeuropea hoy, Madrid/Wiesbaden:Reichert, 113-141.