Joined: Jan 2010
Posts: 9,813
Likes: 1,790
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
That micro-splined inner surface looks like the steel bb shell itself is designed for a press-fit bearing, much the way that some Schwinns had a similar pattern of ribs on the OD of the steerer for the press-fit crown race.
The raised surface reduces the need for close tolerances, allowing for no machining needing to be performed, and so reducing cost.
Not sure though what kind of bottom bracket assembly goes in there, but the ID could be compared to common bearing sizes like 35 or 36mm.