We have inferred the value of $a$ and the positions of the atoms from Figure 2(a) of (Holtzman, 1959) and Figure 3 of (Murthy, 1969), shifting the origin to be on the Mn$_{II}$ atom. The value of $c$ was inferred from the C-Mn$_{II}$ distance given in the paper. This is a pseudo-cubic lattice, with $c/a ≈ 1$.
A. H. Holtzman and I. G. P. Conrad, Structural and Magnetic Properties of Mn‐Co‐C Alloys, J. Appl. Phys. 30, 103S–104S (1959), doi:10.1063/1.2185842.
N. S. S. Murthy, R. J. Begum, C. S. Somanathan, B. S. Srinivasan, and M. R. L. N. Murthy, Ferrimagnetic structure of Mn$_{2}$Co$_{2}$C, J. Phys.: Conf. Ser. 30, 939–945 (1969), doi:10.1016/0022-3697(69)90291-1.
First cited in
H. Eckert, S. Divilov, M. J. Mehl, D. Hicks, A. C. Zettel, M. Esters, X. Campilongo, and S. Curtarolo, The AFLOW Library of Crystallographic Prototypes: Part 4, Comp. Mat. Sci. 240, 112988 (2024). (doi=10.1016/j.commatsci.2021.110450)