Smithing table saved data for saves that used ImmersiveMC 1.5.0 Alpha 2 will be of size 4, even on Minecraft versions before 1.20. This should be corrected in load if this size 4 is detected (set numOfItems to 3 AND shift items 1 and 2 to 0 and 1 respectively*, before calling super()).

*: We should only do this shift if slot 0 is empty (else the table has been used since and thus doesn't need conversion) and slots 1 and/or 2 contain some item (else we don't need to shift).