He means the trigger guard, magazine and floorplate.
On the 1909 Argentine Mauser and the 1904 Portuguese Verguerro, the "bottom metal" fits a standard Mauser 98 action and has a factory-hinged floorplate with a tongue in the trigger guard acting as a latch.
The Argentine floorplate is preferred as it has no capture screw, while on the Portuguese unit, a smith would have to weld over the capture holes to make the unit look commercial.
You also have to recontour the trigger guard, flush off the latch tongue, and braze on a serrated button. You typically "waist" the rear of the guard and shave a few thou off the thickness of the floorplate to trim it up (it's over-beefy). Then you generally weld over the serial number and dress with a file & emery paper, or shave a couple thoug off the front tongue to remove the markings.
Depending upon caliber, you need to extend the mag box as these boxes are shorter than the standard 98 mag box and won't take a .30-06 round, though the action will. Smiths often lengthen the box 1/4" or so by cutting and re-welding.
It's a great unit, but the work is often in excess of $100 to make it the equal of a commercially made part for sporting rifle purposes.
OR.
You just use non-hinged bottom metal and be done with it
