Earlier quoted context omitted.
Counterspell UU Interrupt Counters target spell. Uncommon
Ah yes, the good old days when we got to argue over the fine points of the differences between an "Interrupt" and an "Instant" effect.
Interrupts resolution point which happened within the casting time of Instants or Sorceries. So you could cast an Interrupt "while" an Instant or Sorcery was still casting, but Instants (or Sorceries) had to wait for the current spell to finish casting.
This was of course all ditched when MtG switch to a last in, first out stack model with all instants.