Gearbox identifaction

[Frosty]

I'm about to change the gearbox in my van. The markings on the box I'm about to put in are,
On top ZL in large letters painted in yellow
Right side HOLDEN 10 9938391
Left side 29LS 7 HOLDEN
Front HOLDEN 9930912
The original box in the van at the moment is a M20. Is this a M20 or M21. Can you even tell from the outside?

[HK1837]

They are the same thing. ZL is the broadcast sheet code but box may have been rebuilt. Pull the side cover off and read the numbers on the cluster.

[Frosty]

ZL is the broadcast sheet code

Broadcast sheet code for what?

[HK1837]

For the gearbox. The broadcast sheet would have ZL in the box for gearbox, it is how the people assembling the cars knew what box to put in the car. But as all aussie 4spd's are the same bar a few gears to adjust the ratios the box could have been rebuilt as anything.