In Matsui's paper, Section 5 ‘Linear Approximation of DES Cipher’, Matsui gives two similar equations: Eq. (6) for the first round and Eq. (7) for the third round. The two equations give an approximation of bit $K_1$ and bit $K_3$. Then he derives in Eq. (8) from Eqs. (6) and (7) an approximation of $K_1 \oplus K_3.$
From what I understand, the 3-round DES linear approximation is supposed to give an approximation of only one bit of the key, but here I see two bits from the partial key.
Which bit of the actual key does this approximation refer to?