Near the outbound magnetopause crossing during the first encounter of Mariner 10 with the planet Mercury on March 29, 1974, large intensity, ~6 s quasi-periodic variations in the intensity-time profile of the charged particle experiment's electron counting rate appeared as a series of peaks and valleys. The peaks have previously been interpreted as quasi-periodic burst sequences of mildly relativistic (>175 keV) electrons, caused in one case by episodic ~6-s magnetotail substorm reconnection events and in another case by multiple encounters with a substorm energized electron population drifting around Mercury with an ~6 s drift period. In this paper, we offer a new and fundamentally different interpretation of the Mariner 10 energetic electron, plasma electron, and magnetic field data near the outbound magnetopause at Mercury 1. We show that magnetosheath-like boundary layer plasma was observed up to ~360 km (~0.15 RM) planetward of the dawn magnetopause crossing as sensed by the magnetometer. We show that observations of substorm enhanced >35 keV electron flux (that previously interpreted as >175 keV electrons) associated with the hot tenuous plasma sheet population were interleaved with ~6 s period observations of a cold dense boundary layer plasma associated with a much lower >35 keV electron flux. We argue that the ~6 s temporal signature is due to variation of the thickness and/or position of the boundary layer plasma population. This explanation of the ~6-s variations, based upon our analysis of the coincident responses of the magnetic field experiment and two independent charged particle instruments (at their highest temporal resolutions), finds a direct analogue in observations of Earth's magnetospheric boundary layer, although the time scales are significantly shorter at Mercury. ¿ American Geophysical Union 1989