Taking another look at §163(d), it is also peculiar that the operative word used in paragraphs (1) with regard to limitation and (2) for carryforward of disallowed interest is "allowed" as opposed to "allowable", which appears in the general definition of "investment interest" under paragraph (3)(A).

In the years S/D is claimed under §63(c), deduction for investment interest may have otherwise been "allowable" but it would, definitely, not have been "allowed", hence, subject to carryover.

The use of the words "allowed" and "allowable" are not interchangeable and I remember that the distinctions between these two operative words had been brought up in various rulings.

[Edited: Inadvertently referenced S/D as I/D]