Well I looked up NAPA with the FS-6603 and they state it is in fact a KS6603SB and yes I just replaced mine due to my 2003 HR was cranking with no start. I would have to cycle my switch 3 or 4 times before the engine would start, imeadiatly after replacing the switch it now fires "RIGHT NOW" warm or cold! FYI that switch is what was used for years on Chevy pick ups from 1969 to like 1976ish. When I bought mine it was like $23 and change... by far the least expensive item I have probably paid for on my RV. Good Luck Stay safe.
2003 Holiday Rambler Endeavor 36 Ft.
2019 Can-Am Maveric X3 Turbo 120 REALLY fun HP😁