The first mention of the 511 oscilloscope appeared in the April 1948 issue of Electronics.  It wasn't called the 511, simply the Vollum oscilloscope.

 

The first advertisements for the 511 appeared 5 months later in September 1948. This ad is from the September 1948 issue of Electronics.

 

This similar but a slightly different ad for the 511 oscilloscope appeared in the September 1948 issues of Proceedings of the IRE and The Review of Scientific Instruments.

 

This ad for the 511 appeared in the October 1948 issue of Proceedings of the IRE.

 

This ad for the 511A appeared in the November 1948 issue of Electronics .

 

This ad for the 512 appeared in the December 1948 issue of Electronics and Proceedings of the IRE.

 

This smaller ad for the 512 appeared in the December 1948 issue of The Review of Scientific Instruments.