How many versions of the P-51 dorsal fin fillet are documented?
A quick search of Google Images seems to show only three major variations on the tail for production P-51 variants:

Razorback (all P-51A, B, and C variants; no dorsal strake and a higher rear fuselage line):



Bubble canopy without the strake (seen on early D-variants)



With the strake (seen on later production D variants, integrated fully into K and H models and retrofitted to earlier Ds, sometimes to the Razorback B/Cs):







There is some difference in the prominence and exact shape of the fin fillet as the design continued to be developed; for all B/C/D variants it was a thin piece essentially bolted onto the strakeless frame. For the H variant in the bottom picture, it was integrated more fully into the fairing of the vertical stabilizer.

There's one more I've seen and I'm not sure if there's just the one example, but there is a P-51 "XR" variant, "Precious Metal", with dual counter-rotating props and further aerodynamic refinement, including a fairing behind the modified bubble canopy that narrows into a continuous strake along the aircraft's rear spine:



Other custom racing variants have existed over the years, all heavily modified from USAF production variants.