ideally, you'd want to look in the manual for a part number... or on the part itself and see if it has a part number stamped on it. using the part number in a search is going to be your best bet to find a weird part like that.
that said, it IS likely used in a few machines, which... i don't know... rowe does like to recycle parts as much as possible...but just searching for a CD-51 title page knob doesn't immediately show anything up...
but... searching around... replacement title pages themselves seem like they were used in the CD-100 A/B/C/D/E and the 51 series jukeboxes so maybe the knob is the same or similar as one used in one of the CD-100's.
good news is, I do have a pile of title page mechanism parts for CD-100 series jukes (we converted piles of CD-100 jukes to digital touch screen jukeboxes in the mid 2000's) so if it happens to be one used in of those models, i'll have it somewhere, cause i have dozens of boxes of parts from them... so if you can get a part number for yours... i'll search the manuals for CD-100A and up and see if it's used in any of the CD-100's.