Hi Deepak,
If you are doing with MB1B, I am not sure how you are doing against production order as this is a transfer posting transaction and in standard doesn't carry 261. You should use MB1A and with respect to production order. If don't have backlflush indicator still it would be visible in COOIS when you do with this.
In COOIS select "object overvierw" in list field, put production order number and see the results.
Let me know if it resolved the issue.
Regards
Rahul