Idler pulley question

[JLEONARD]

OK. Time for my stupid question of the week.
I ask because my son needs to set up his 89 CTD that's now resting in the S-10 pickup.
Motor is an 89 Cummins out of a D350. The engine had A/C but he removed the compressor since there's no room for it or the system. Right now the rest looks exactly like my 91.
The belt routing shows with A/C the belt contacts the idler on the non grooved side.
With no A/C the belt contacts the idler on the grooved side.
The new idler he got (old was shot) is smooth.
Mine has shallow grooves. Is it supposed to have grooves? Or is mine worn that way?
Dealer and parts stores tell him that the smooth idler is the only one listed.
I recall noting that the one on my boat engine was grooved when I looked at it when it was new.

So...should it be grooved? Or smooth? Are the idlers the same for both?

Thanks.
E C Jay