And here we have it: The result of the interrupt test program on my real MC-1000:
Code
CALL 16128
......1......1...............1..
1..1......1...............1.....
.....1................1......1..
2......1......1...............1.
1......1......1......1......1...
.......1......1...............1.
2......1...............1.......1
..........1.....1......1......1.
.1.....1......1.......1.....1...
2...
OK
CALL 16128
...............1.....1.....1....
1........1.....1.....1.....1....
1.......1..............1....1...
2.1.....1.....1.....1......1....
1...............1...............
2......1.....1.............1....
2.........1.....1.....1.....1...
1........1.....1.....1.....1....
1.......1......1.....1.....1....
1.......
OK
It's a little more erratic than I expected*, but there are the predicted 1's.

(* Why do 2's appear sometimes, and also consecutive interrupts too close to each other?)