Briarberry, quiltbuddy,
Look close at the 99 in the pic above. See the round chrome nob just above the badge, that is the stitch length control nob. Ne reverse or back tack feature.
Now, look below at the pic of my 99K. See the stitch control lever. All the way up ^ is reverse or back tack, down is stitch length control.
The machine above is the original type and pre-dates the back tack or reverse features.
I'm with Charlee, for that price grab 'em both.
Joe