Here's my thoughts......
Two auld lads were drinking in a pub somewhere in North West London one Friday evening, possibly second generation Irish.
As the drink flowed so did the shite talk, and the inevitable "do you know what I reckon would be a great job" and so the grumper idea was conceived.
As the Gunness turned to Jameison the sketches started on the back of beer mats and the crazier and more complex the design got.
Now around about 8.30 the young Polish fitter/welder that works for one of these aul lads walks in. He is swiftly called over and shown the latest design and told, tomorrow we want you to take that old dumper down the yard that was rolled and make this! He shrugged his shoulders and Downed his last vodka and heads home as he has a busy weekend ahead.
7.30 Monday morning aul lad no. 1 opens the workshop door to a strong smell of paint fumes and a bright orange "grumper" in front of him.
Feck me he thinks and goes a books a plot at plant works straight away.
Now all he has to do is watch the orders roll in