Also, if you are interested in the mythology of Janus, I cannot recommend highly enough this article by Rabun Taylor. If you can't get access I have it somewhere but it will take me awhile to get to. Rabun is a great scholar and this article is one of his best.
What puzzles me, is the weight and date of the Janus As in the OP. Shouldn't an As of 200-190 BC weigh approx 40 gram instead of 6 gram (adding some weight for the missing part)
Not necessarily - 20 gram asses were struck in Sardinia, Sicily or Southern Italy during the second Punic War(cf. McCabe's paper on anonymous bronzes from Essays Russo, group H1) so we know that bronzes struck for use elsewhere were not necessarily struck to Roman weight standards standards. These may well have been struck at some standard already in-use by the locals and meant purely for local circulation.
It is how I understand it also. Similarly, Rome minted Litrae for So Italia trade, but the Quartuncia was reserved for Roman circulation. Parallel mintages for local accepted markets.