Seems odd fitting winglets to 733s when they have several 752 and 738s without winglets. Maybe the fuel burn advantage is better on these or the kit is cheaper. It is hopefully a sign that they intend to fleet fit them.
Spot-on, the Classic CFM 737s are pretty under-winged and the winglets give them a fuel-burn improvement right off the bat ( from routes 200 nm upwards I think ) whereas the NGs need to be operated on longer runs to see a similar benefit, as their wing is much improved as a baseline.
The Classics are also much lighter than the NGs, for the '300 versus '800 the empty weight difference is in the region of 10 tonnes! So they're much better suited to short-hop routes than the big heavy NGs.
Edit: actually this APB chart shows fuel savings almost immediately for the '300
737-300 | Aviation Partners Boeing