I have now tested a US based MC in addition to my Visa. The MC was used for a ARS 9500 purchase. Initial charge was $53.22. Credit 3 days later was $22.42. Net charge was $30.80 for an effective rate of ARS 308 / USD. So my Visa is getting a slightly better rate. The visa continues to post at a rate of about ARS 312 / USD.