SJ Radar - speaking from the "system" point of view - is available June 1st, 1942 on the Salmon / Sargo (as well as others). However, the outlying locations often get the new gear later, and that part can be unpredictable. Had you gotten it on the S-Boat prior to the upgrade? btw, There is a "SubmarineAndEquipmentAvailabilityChart.pdf" file in the Support folder of the mod / game, which is improved for the next version (though already out of date)
sigh