From Petros Hadjicostas, May 25 2019: (Start)
This is column alpha = 2 in Table 5 on p. 1179 of Cyvin et al. (1994). (The table starts with column alpha = 0.) Here, the index alpha for the columns in Table 5 is the number of appendages to the core.
The table refers to the example described on pp. 1177-1179 of the paper and especially to Figure 5 (p. 1178). The entries refer to the number of nonisomorphic systems of catafusenes in this example (as shown in Figure 5, p. 1178).
The rows of Table 5 are indexed by the total number of hexagons in the appendages.
Therefore, a(n) = number of nonisomorphic systems of catefusenes in the example on pp. 1177-1179 of Cyvin et al. (1994) with alpha = 2 appendages to the core, where n = total number of hexagons in the appendages.
From Note 10 (p. 1180) in Cyvin et al. (1994), we see that (most probably) the number of hexagons a (= n in this sequence) does not include any hexagons in the core.