I just added a diode! Can you work out where?

Shahriar from The Signal Path channel, in his video #202 about the HP5245L frequency meter, remarked about how cool it looked when it showed the Nixie tubes counting up to the frequency being measured. • TSP #202 - A Uniquely ...
I agree that it looks pretty nice, so I wanted to make my Yaesu YC355D do something similar.
But in my previous video about the Yaesu, I was slightly mistaken in my understanding of how it worked, which would have made this modification quite a bit more complex.
When I realized my error, the mod became as simple as just adding a 1N4148 diode.
Here is the schematic, can you work out where the diode went?
imgur.com/kbtQiBq

[@ricksampson6780]

I have no idea, I just like old discrete IC tech!

[@bertoid]

Glad you do!
(I've added a comment above that makes it easier)

[@bertoid]

Not really a fair challenge!
I would never have thought of the mod looking at that diagram, because it shows the IC's as DIP packages, rather than as logic elements.
But the manual includes this diagram, which makes working out how to make this mod much easier!
imgur.com/HA4z5DL