| System | Period | Rows | What it dates |
|---|---|---|---|
| C.V. Heljestrand — the designation “MK” (a model, not a year) | the years of the firm are not publicly documented; the Eskilstuna school — the nineteenth and twentieth centuries | 5 | No “MK number → year” key exists: in the guide “MK series” is the firm's TRADEMARK, not a date. There are no years of operation for C.V. Heljestrand i |