Crossed version: the photon absorption order is swapped.
The computational chain
From diagram to number—every step of the amplitude calculation.
The diagram
The photon absorption order is reversed compared to the t-channel: photon $q_2$ is absorbed first, creating a virtual electron with momentum $k = q_2 - p'$ that then absorbs photon $q_1$ and produces the outgoing pair. The propagator carries $k^2 = u$.
Feynman rules expression
$$i\mathcal{M}_u = (-ie)^2\,\epsilon_\mu(q_1)\epsilon_\nu(q_2)\,\bar{u}(p)\gamma^\nu\frac{i(\not{q}_2-\not{p}'+m)}{(q_2-p')^2-m^2}\gamma^\mu v(p')$$
This is the t-channel expression with $q_1 \leftrightarrow q_2$. The fermion line runs from the positron ($v$) through the two photon absorption vertices to the electron ($\bar{u}$), with the virtual electron propagator carrying momentum $q_2 - p'$ instead of $q_1 - p'$.
Relation to pair annihilation by time reversal
$$|\mathcal{M}(\gamma\gamma \to e^+e^-)|^2 = |\mathcal{M}(e^+e^- \to \gamma\gamma)|^2$$
The squared amplitude for pair production equals that of pair annihilation with the same Mandelstam variables — time reversal (CPT) guarantees this. The u-channel of pair production is the time-reversed u-channel of pair annihilation. Cross sections differ only through the flux factor and phase space.
Mass regulates the amplitude
$$\overline{|\mathcal{M}_u|^2} \supset -2e^4\frac{m^2 - s}{m^2 - u}$$
The propagator denominator $(m^2 - u)$ vanishes in the massless limit, so the electron mass cannot be dropped.