Hi Kunal,
The lock will appear in production system only as the standard logic checks for production client before showing up the lock.
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CHECK g_clnt_prod = 'P'.
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Following two FMs are used to determine whether the locked icon is to be displayed or not
ISU_DB_EVER_FIND_CONT_FOR_PORT,
ISU_DB_TE422_FIND_MRU_FOR_PORT
Simply put forward, if portion is being used for an isu contract(EVER) or is associated with any MRU(TE422) lock icon will be displayed.
I hope, this clarifies you doubt.
Thanks,
Ritesh