Plarail PR Tankers?

Does anyone know anything about these tankers?  They weren't a catalogue item (Edit: yes they were, see below!). The green one is described as a 'Toyama Oil Transport Co.' wagon.

  

[Super]

Plarail Yard Full of Freight Set supposedly released for a short time in 2001

Brilliant! Thanks Super. I'd completely forgotten about that set. I just picked up three of the silver tankers, hence my question.

Clearly I didn't examine the catalogues closely enough...

2001-2002:

2002-2003:

Then replaced in 2003 by the EH500-10 Kintaro set:

[Super]

What I find curious is that all the vehicles in this set, including the engine, have yellow wheels but the green caboose has red.

The EH500-10 Kintaro set also has exclusive Tankers but in this case there are 3.