The assignee ranking is led by a single company with 524 records, well ahead of fifth place at 199 and tenth place at 63. The top five assignees together hold 1,793 records, 42.5% of the 4,222 records in scope, and the top ten extend that to 57.7%. This is a leader-plus-long-tail structure: one dominant filer, a handful of strong followers, and then many companies with only a few filings each.

Filings rose from 124 in 2017 to a peak of 211 in 2021, then fell to 101 by 2024, a 52% decline over that three-year window. Figures for 2025 and 2026 are lower still, but publication typically lags filing by about 18 months, so those recent years are undercounted and should not be read as proof the field is dying. The safest complete read is the 2021-to-2024 decline; anything drawn from 2025-2026 needs revisiting once more filings publish.

Beyond core iron-production claims (C21B, 44.1% of records), the dataset shows meaningful overlap with refractories and cement (C04B, 16.2%), metal extraction and refining (C22B, 13.3%), and furnace hardware (F27B and F27D). Smaller but notable shares appear in layered products (B32B), material testing (G01N) and building structures (E04B), reflecting how furnace lining, sensor instrumentation and structural design all intersect with charging and layering technology. A single filing often carries several of these classes at once.