It all boils down to couplet 36 of Bolton's (1987) key. I think it would be possible to debate for hours over whether the sculpture on the head of this specimen is 'uniformly densely reticulate-punctate' or 'with a silky, smeared or roughened appearance'. Initially I opted for the former because, well, it is reticulate-punctate, just more finely than the likes of Monomorium bicolor! However, I've since come to realise that this is what Bolton means by the latter, which leads to M. dakarense.
The difference between M. dakarense and M. dictator, other than the ambiguities of cephalic sculpture, seems to be size, number of ommatidia in the longest row and, importantly, scape index (SI). M. dakarense is overall the smaller of the two, with fewer ommatidia and a SI of 95-100, whereas M. dictator has a SI of 107-109. For those not familiar with SI, it is as follows:
Scape length × 100
The SI in this specimen is 92, putting it closest to the range given for M. dakarense. Given that Bolton only had the three syntype workers to measure it is likely that the true range of the SI includes 92. However, this is not the only difference between this specimen and the holotype, as the other measurements are (quoting the measurements for MJL143, followed by the range given by Bolton, in mm):
Head length = 0.61 (0.57-0.59)
Head width = 0.51 (0.44-0.47)
Cephalic index (ratio of head width to height) = 84 (77-80)
Scape length = 0.47 (0.44-0.45)
Pronotal width = 0.31 (0.30-0.31)
Mesosoma length = 0.72 (0.66-0.70)
Despite the larger size, the specimen is not big enough to be M. dictator and differs in the other features mentioned, so it is most likely M. dakarense.
It still remains a bit of a rediscovery. M. dakarense had been collected once in Senegal in 1914, so it's more likely than M. dictator, which had only been collected in Angola. The only specimens that I collected were found on the ground at Madiyana Camp on Jinack Island, and only at night. Since we stayed at Madiyana Camp for a few days it would be surprising if M. dakarense had been active during the day and I had not collected it then, which indicates pretty strongly that it is entirely nocturnal. Perhaps a combination of its nocturnal habits and its small size have meant that it has not been found again until now?