Here's a paper:
Toll, R.B.; Binger, L.C. 1991. Arm anomalies: cases of supernumerary development and bilateral agensis of arm pairs in Octopoda (Mollusca, Cephalopoda). Zoomorphology 110: 313-316.
The abstract goes as follows:
The first case of true hexapody among the Octopoda, resulting from bilateral agensis of one arm pair, is described for a male Pteroctopus tetracirrhus. A case of decapody, with uniform development of all arms, is also reported for the first time for a male Octopus briareus. Both conditions apparently result from developmental anomalies of the embryonic arm anlagen. A survey of other anomalous conditions relating to arm devlopment and regeneration within the Cephalopoda is provided. A possible relationship of polyfurcation of arm tips in the Octopoda with regenerative processes in amphibian limbs leading to similar conditions is suggested.
They cite the following papers:
Gleadall, I.G. 1989. An octopus with only seven arms: anatomical details. Journal of Molluscan Studies 55: 479-487.
Kumph, H.E. 1960. Arm abnormality in Octopus. Nature 185: 334-335.
Okada, Y.K. 1935. An octopus with branched arms and mode of branching. Annot. Zool. Japon. 15: 5-23.
Okada, Y.K. 1937. An occurence of branched arms in the decapod cephalopod, Sepia esculenta Hoyle. Annot. Zool. Japon. 17(1): 93-94.
Okada, Y.K. 1965a. On Japanese octopuses with branched arms, with special reference to their captures from 1884 to 1964. Proc. Jap. Acad. 41(7): 618-623.
Okada, Y.K. 1965b. Rule of arm-branching in Japanese octopuses with branched arms. Proc. Jap. Acad. 41(7): 624-629.
Palacio, F. 1973. On the double hectocotylisation of octopods. Nautilus 87(4): 99-102.
Robson, G.C. 1929. On a case of bilateral hectocotylisation in Octopus rugosus. Proceedings of the Zoological Society of London, 1929: 95-97.
Smith, E.A. 1907. Notes on an "Octopus" with branching arms. Ann. Mag. Natur. Hist. 7: 407-411.
..........
Sorry about the abbreviated reference citation for many; this is as is in Toll & Binger's paper; I can cite full details shortly (just need to check those Japanese ones out).
Cheers
O