Model number search behavior appears to have been modified at eBay to support partial keywords in any position in the model number:
clevite MS981 will match these: MS981 MS-981 MS981A MS981P20
clevite 981P20 will match these: MS981P20 MS-981P-20
These are examples and are not all inclusive. Other similar numbers may match.
But it looks like matching is based on the full letter and number groupings. Partial groupings do not match:
clevite S981P20 will NOT match MS981P20 because the M in MS is not included.
clevite 81P will NOT match because the 9 is not included.
The search algortithms appear to be treating the model number MS981P20 as MS 981 P 20 which can be searched in combination in order sequence only. e.g. MS20 will NOT match.
Hyphens and dots/periods are ignored and can be skipped or included in search filters as is common in text searches.
Strings of numeric values can be matched multiple ways. Consider this listing title for automotive fuses:
Fuses Assorted 15 pcs 5A 7.5A 10A 15A 20A Pack
Any of the following search phrases would match this listing:
10A 15A 20A
While this has some advantages to doing wildcard style searches, it can also result in false positives.