= Solution
For any indexed cubic reflection,
$$
a=\frac{\lambda\sqrt{h^2+k^2+l^2}}{2\sin[\tfrac12\arctan(R/L)]}.
$$
Use $\lambda_{\rm Mo}\simeq0.07107\,\mathrm{nm}$ for the unresolved <molybdenum> $K_\alpha$ doublet; the tabulated components are approximately $0.0709300$ and $0.0713590\,\mathrm{nm}$. These are external spectral constants, not an assertion about the unavailable historical Data Book.<X-ray wavelengths and X-ray atomic energy levels by J. A. Bearden>[Table V, molybdenum entries; physical PDF page19]{ref}
Reading broad ring centers in the supplied image more closely gives representative <radii> $5.96,6.99,10.63,13.23\,\mathrm{mm}$. Their corresponding estimates are about $0.3471,0.3473,0.3478,0.3480\,\mathrm{nm}$. Their small scatter understates possible common scale and camera-distance errors. In particular,
$$
\left|\frac{\delta a}{a}\right|\simeq
\frac{\cot\theta}{2[1+(R/L)^2]}
\left|\frac{\delta R}{L}-\frac{R\,\delta L}{L^2}\right|
$$
for a first-order signed perturbation; independent uncertainties are combined in quadrature instead. A reading uncertainty of order $0.2\,\mathrm{mm}$, plus the printed-scale and setup uncertainty, justifies a deliberately modest precision:
$$
\boxed{a\simeq0.35\pm0.01\,\mathrm{nm}.}
$$
\b[<Nickel> is a plausible identification], since it is FCC with room-temperature edge about $0.352\,\mathrm{nm}$.<Thermal expansion of nickel and iron, and the influence of nitrogen on the lattice parameter of iron at the Curie temperature by Jinn-Wen Hwang>[Abstract, pure-nickel lattice parameter at25°C]{ref} The source photograph alone does not chemically identify the specimen; nearby elemental lattice constants and the scale uncertainty preclude a rigorous unique assignment. The estimate is not reported with artificial precision inherited from pixel fitting.
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