I believe the hole you have marked Speedo cable is correct.
The hole you have marked throttle and choke cables, also handles the temp sensor and oil pressure line. The grommet handles all four. The hole marked ??? is for the vacuum rubber line to your wiper motor.
The two holes marked temp sensor and oil pressure don't exist on my '51, so I think they may not be original. Let's see what others think.
Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix