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Full length rolled seam first, then partial seam for ease of manufacture and at a lesser cost.
The term hardback with the full seam, later partial seam had the back split folded and spot welded.
More dollars for a hardback full seam 30 round mag. My area $75 used with wear, $100+ mint.
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c east: You asked a very similiar question a while back. Here is the answer to identify US GI 30 round magazines. Maybe one of the moderators will make it a sticky for future reference:
30 round mags - Military Surplus Collectors Forums