This original configuration is commonly known as a "hex receiver". Besides the obvious polygonal shape of the exterior of the receiver itself, it has a tapped and plugged hole next to the recoil lug, a shallow magazine screw hole, stepped tang, extra notch forward of the stripper clip guide, low wall, machined right rear, and inner receiver ring without the lug race cut.
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The round receiver was adopted along with the model M91/30 in 1930, but did not go into production until 1935 and 1936 at Izhevsk and Tula respectively. It retained all the same sub features as the hex receiver (#1); shallow magazine screw hole, stepped tang, extra notch forward of the stripper clip guide, low wall, machined right rear, and inner receiver ring without the lug race cut. All future Mosin receivers are round and no hex receivers are known to have been produced after 1936. Any rifles or carbines with a hex receiver and a barrel date after 1936 were built on older receivers. One exception is a few 1937 rifles built on 1936 receivers.
Later in 1941 Izhevsk carried the streamlining process further by bringing back the high wall receiver for all rifle and carbine production, not just snipers. At this time the right rear was no longer machined. The left rear continued to be machined because it is necessary to hook the cocking knob on it to place the rifle on "safe". The extra notch forward of the stripper clip guide was was still in place, although apparently for a brief time, as this variation is uncommon.