The regular SPM 1.5 unit list does not include any playable Panzer IIIJ L/42. But it is possible to 'unlock them' with additional entries in the 'common_units' and 'common_res' files.
Why dou you need to modify common_res?
The shell entries were linked to the icons for the user interface:
PZGR_50,0x01,%SHELL_AP%,KWK38,683,40,0.5,0.1,0,1,1,0,2.06,0,0.2,8,1,1,0.1,0,1,1,1,
expl_gndl*,expl_techl*,expl_buildl*,expl_water*,expl_human*,fly1,0,1000,1,txt_shell_pzgr_50,
ap_expl_pat,rico_pat,piece_pat,0,1,1,1,1,0xffff4040,3,1,
21,SHELL_MED,SHELLC_MED;
PZGR402_50,0x01,%SHELL_APSC%,KWK38,1130,40.4,0.7,0.5,23,1,1,0,0.9,0,0.5,8,0.5,0.5,0.1,0,1,1,1,
expl_gndl*,expl_techl*,expl_buildl*,expl_water*,expl_human*,fly1,0,500,1,txt_shell_pzgr402_50,
apsc_expl_pat,rico_pat,piece_pat,0,1,1,1,1,0xffff4040,3,0.7,
22,SHELL_MED,SHELLC_MED;
Just a little something to make things more presentable,