Old Peace Walker logo shows it was originally Metal Gear Solid 5
There it is. Metal Gear Solid: Peace Walker is Metal Gear Solid 5.
Or at least it used to be.
A posting on the Kojima Productions developers blog from PR Jiro Oishi shows the game was seen as the next main canon title in the series while planning the title.
It was, however, dropped afterwards, just before its reveal at E3 last year.
It's not first confirmation the title was meant to be MGS5, though.
Hideo Kojima confirmed a couple of weeks ago that the game was considered to be a main title in the series, although because it's on the PSP, it won't be a numbered title in name only.
Megaton. Find out more through here. Peace Walker is due for release in May in the US and Europe, with a launch in Japan in April.