The 20ATZ is the sister of the famous 20AT. Both machines are the first models built with the Register IV (Triplex). See the description of the 20AT for more details. They are the only black models with this very complex carriage.
We can point out two important differences between the 20AT and the 20ATZ:
This model, like all machines with the letter “Z” has a tens-carry which extends to the full capacity of the Product Register. The 20ATZ has tens-carry-over to two dials to the left of that aligned with the extreme left-hand column of keys. On the picture below, we see the typical round sensors which appear on the left lower part of the carriage.
On the other hand, the 20ATZ has not the second Counting Register that we find on the 20AT (see the page of the 20AT). This additional Register is present only on the 20AT and the 20BTG. Furthermore, these essential discrepancies between the 20AT and the 20ATZ are the same between the 20BTG and the 20BTZG.