During the long production period Zeiss Ikon changed the naming convention of the Ideal (and other types of folding camera's). The last range of camera's had the following numbers (these camera's had
more rivited parts in stead of screws/bolts):
camera > plateholder
250/3 (6.5 x 9cm),
250/
7 (9 x 12cm), > 726/
7
250/
9 (10 x 15cm) and
250/
11 (13 x 18cm).
The earlier models (just after the merger with ICA in 1926/27) had a different numbering taken from ICA:
camera > plateholder
111 > 726/3 (6.5 x 9cm),
246 > 726/
6 (9 x 12cm),
225 > 726/
6 (9 x 12cm), older model camera: 205
325 > 737/
12 (10 x 15cm),
385 > 737/
13 (13 x 18cm),
651 > 726/4 (6x13cm stereo) and
650 > 726/10 (9x18cm stereo).
(the Ica catalogue from 1919 mentions plateholders with the number 72
7/6 for the Ideal 9x12 cameras (the 205 and 246), but this seems a misprint since the pages with the different holders only mention nr. 72
6/6.)
The 225 and 246 series for 9x12 format take the 726/6 plateholders (728/6 for filmpacks)
I don't know whether the 250 serie takes the same format plateholders as the early series, but to go safe I would opt for the 726/7 holders.
The system is not always as straightforward as it might seem at first site: e.g. I had the ICA and Zeiss Ikon favorit folding camera, which were identical but could be choosen with different backs; one took slide in plateholders, the other the so-called pop-on pop-off holders.