[oysterman23]

If this curved foot section has sail track slugs attached, is 'curved', its probably what is known as a 'shelf foot'.
Its purpose was to allow for full draft to be carried down well into the lowest panel sections of the sail and to lessen air flow across the foot from the high pressure side to the low pressure side.

Shelf footed mainsails are from a time when it was thought by most racing sailors that FULL draft was beneficial in light winds and from a time when most racing sailors sailed dead down wind (on purpose and with all sorts of auxiliary downwind sails - bloopers, tall boys, spinnaker staysails etc.) - in effect its a mainsail that is optimized for sailing dead down wind.
(Modern aerodynamics theory as applied to sails (~mid 1970s) has proved this to be very inefficient. Most of what most folks learned in a high school 'science' class about what creates lift and how wings/sails work is ... dead wrong!)
Such sails usually were also constructed with a flattening reef ... and there may be reefing cringles a foot or two above the foot on the leech and luff.

It's still useful and just as efficient as a modern 'loosefoot' mainsail; although its more difficult to tension and adjust sail shape in the foot section, usually needs a multipart block system on the outhaul.

Nicely said!
One of my mains is built this way....the "shelf" was often offered with a zipper to dump air when wind strengthened...
The flattening reef is actually a nice feature particularly when the sail is older ...im looking forward to the new loose footed main on the way!....