Validation of Elemental and Isotopic Abundances in Late-M Spectral Types with the Benchmark HIP 55507 AB System

M dwarfs are common host stars to exoplanets but often lack atmospheric abundance measurements. Late-M dwarfs are also good analogs to the youngest substellar companions, which share similar T-eff similar to 2300-2800 K. We present atmospheric analyses for the M7.5 companion HIP 55507 B and its K6V primary star with Keck/KPIC high-resolution (R similar to 35,000) K-band spectroscopy. First, by including KPIC relative radial velocities between the primary and secondary in the orbit fit, we improve the dynamical mass precision by 60% and find M-B=88.0(-3.2)(+3.4 )M(Jup), putting HIP 55507 B above the stellar-substellar boundary. We also find that HIP 55507 B orbits its K6V primary star with a=38(-3)(+4) au and e = 0.40 +/- 0.04. From atmospheric retrievals of HIP 55507 B, we measure [C/H] = 0.24 +/- 0.13, [O/H] = 0.15 +/- 0.13, and C/O = 0.67 +/- 0.04. Moreover, we strongly detect (CO)-C-13 (7.8 sigma significance) and tentatively detect (H2O)-O-18 (3.7 sigma significance) in the companion's atmosphere and measure (CO)-C-12/(CO)-C-13=98(-22)(+28 )and (H2O)-O-16/(H2O)-O-18=240(-80)(+145) after accounting for systematic errors. From a simplified retrieval analysis of HIP 55507 A, we measure (CO)-C-12/(CO)-C-13=79(-16)(+21) and (CO)-O-16/(CO)-O-18=288(-70)(+125) for the primary star. These results demonstrate that HIP 55507 A and B have consistent C-12/C-13 and O-16/O-18 to the <1 sigma level, as expected for a chemically homogeneous binary system. Given the similar flux ratios and separations between HIP 55507 AB and systems with young substellar companions, our results open the door to systematically measuring (CO)-C-13 and (H2O)-O-18 abundances in the atmospheres of substellar or even planetary-mass companions with similar spectral types.