I can supply copies of the Russia/Austria postal treaties you require, all except the 1850, which is not known to me. The July 1849 convention became effective on 1 January 1850 (Western calendar) so, is it possible that your 1849 and 1850 treaties are one and the same?
Are you attending the London meeting in October? If you are, I could hand over the copies to you there. In the meantime,if you have any specific queries, please e-mail me: firstname.lastname@example.org – and I may be able to answer them, subject to my ability to translate the originals.