Some models of PS/55s are described as xxx0 and at the same time these models are also described as xxx1. For example, model "5551" is sometimes called as "5550", "5561" is called as "5560" and so on. What is the trick (or logic whatever) there? I am not sure about the logic or trick. I may use 5551 in some case and I may use 5550 in another occasion. So please don't think these are different models.
Model 5530, 5510, 5580 These all do not have xxx1 number. They are always described as xxx0. On the other hand, 5550, 5570, 5560, 5540 these models have xxx1 number with specific type-suffix. (Recently I noticed that 5560-N has also 5561 number with type suffix). Really strange... There is NO 5520 number for model 5521-Y.
Wild assumption for 0 and 1 and model coding
During email transaction with Mr. Tahara who knows a lot about IBM Multistation 5550 series We reached following assumption about numbering of PS/55.
Model numbers of Multistation series and PS/55
PS/55 Model Numbering "Rules"
xxx0 was used as general number indicating "system" as a whole and
at the same time it was used as model number.
The rule might be explained as follows.
For desktop models (Including tower model 5570)
Server model (5580)
Server model was a *new* segment which had not been available in Multistation series. They no need to follow existing numbering rule. Thus 5580 was always 5580 like 9595 and 9585/8595.
Entry class machine
Figure 1 to 3 for 3rd digit ( xxNx ) was kept for *Entry class machine such as xx10, xx20 and xx30. xxx1 was not used for *Entry class* machines. Model 5510 and 5530 has no xxx1 number even for any specific types. Exception was xx20 for 5521-Y.
The last desktop model derived from 5550-R/L (same body, almost same planar).
Why it was not coded as 5551-xxx then?
According to PS/55 coding, 486 model were coded as:
Y should be suitable for i486SX/33 but 5551-Y was already marketed. Alphabet left unused were "I", "O", "P", "Q", "X" and "Z". IBM didn't use "I", "O" and "Q" because these alphabet might cause confusion between alphabets and numeric "X" and "Z" were supposed to be suitable for "High end" (or Ultimate) machines but not for Entry level machine and these letters might be kept for future coming models. And anyway it has 486SX33 for which "Y" was defined. Thus IBM had no choice but name it as 5521-Y.