When did Black and Decker stop making the WorkMate using cast aluminum?

The first mass-produced Workmate had the cast aluminum "H" frame, and is known as the model 79-001. It appears that up to 10 different types of the 79-001 were introduced worldwide. For example, the type 1 had the largest number of cast parts and sported an orange wood top. The type 2 (which I happen to have) had fewer cast parts and an unfinished wood top. The type 4 is also quite common.

Model 79-001 Workmates sold in the USA were manufactured by Black & Decker in Brockville, Ontario, Canada. According to an article in the August 1975 issue of Popular Mechanics, the 79-001 was sold in Canada and England before being introduced to the US market in 1974. The article lists the retail price as $89.99

According to page 219 of "The Workbench Book" by Scott Landis, about a year after the 79-001 was introduced in the USA, Black & Decker decided to replace the aluminum "H" frame of the units built in Canada with stamped steel. Units built in England however, retained the aluminum "H" frame for several more years.