The datasheet for the 215SP series (linked here) has a space in the part number for packaging options, however the datasheet only shows the “X” filler instead of any specific packaging codes. For other packaging codes see the following document from Littelfuse: Part Number Structure
For an example of how alternate packaging codes fit into the numbering for these specific axial fuses, the packaging table on page 4 of the 215 datasheet illustrates this well: 215 Datasheet