Early examples used cast steel fittings with the tubing brazed in.
Later models are of the same design but of welded construction.
The latest models have done away with castings and substituted pressed steel reinforcements. All are repairable within limits. The rigid frames are often cracked through the seat post at the bottom engine mount. Often there have been many attempts to make repairs with arc welds and gussets. Careful inspection will invariably find cracks in the welds.