Plomb was a major tool manufacturer founded in Los Angeles. It was the predecessor of Proto Tools. As the company expanded and acquired factories elsewhere, they changed the location name on their tools in 1939.
Early 1939 – “9” followed by a letter. The “O” in Plomb was represented by an upside down triangle, and the tools in Early 1939 were stamped Los Angeles.
Late 1939 – “9” followed by a letter. The “O” in Plomb was represented by an upside down triangle, and the tools in Late 1939 were stamped “Made in U.S.A.”
Information and images from http://toolarchives.com/book/export/html/4?Plumb-Proto%20catalogfield_model_value=&field_size_end_1_tid=&field_wrench_overall_length_tid_op=or&field_wrench_overall_length_tid=&&sort_by=field_date_code_value&sort_order=ASC&page=10 . Download the 1939 Catalog # 17B catalog.