Hi Andy,
There are two conflicting aspects of this enquiry. Rockwell existed 1907-20 (and possibly later). The 1st Czechoslovakian Republic did not come into existence, at least on paper, until October 1918, hostilities ending on 11th of November, and the official state and constitution in 1920. The "Made in Czechoslovakia" mark would have been possible from 1919, but I have no date for its first use. Is it etched?
The signature is very faint, and I wonder if you can manage a picture from a slightly different angle? Difficult to say loetz...
It is clear however that "Tango" is a fairly generic description of this glass, and not all "Tango" was designed by Powolny, or indeed, made by Loetz, and other prestigious makers such as Harrach (Harrachov) are certainly known to have had a "Tango" range, and to have had craftsmen and women, who could have carried out the silver decoration to the pieces that you have.
I take it that the silver fittings are all marked Rockwell?
Regards,
Marcus