Here's where I would look for it first.
Sort the OKWIN listing by talkgroups in numerical order by clicking List all in one table.
Find GRDA Police which is 64048.
Notice that the ones next to it are 64144 BIA PD NE and 63984 Pryor Fire.
They may have a block of consecutive talkgroups.
I would look for them on 64016, 64080 and 64112.
I would suggest monitoring OKWIN with Unitrunker, see which radio IDs are associated with their primary talkgroup, and see which unidentified talkgroups they are logging into/using. That would be a good place to start.