This short, dense well illustrated article is pretty convincing.
I want to think about what it means for other janiform heads on other (Non Roman) Janiform heads, esp. the third century aes grave of Volterra (Vecchi 128-144). Vecchi thinks Roman types may be influencing Volterran choice in this instance.
235 BCE makes sense as a watershed date for the quadrigati and the janus/prow series, but it still leaves me wondering about the as of RRC 14 which Molinari and Jaia help put into a pre-Punic War context.