This got me thinking and a quick look on eBay found me this version instead:

The part code is 0-609-25, and it's available on line from various car parts suppliers.
The lens has a fine cross-hatch pattern:

and a slightly domed profile:

It's slightly too wide to fit in the Armortek part, so a quick whizz round with a file took it down to the right diameter. Then the lens was parted from the body and cleaned up with a craft knife:

And hey presto, a Tiger reflector:

Adrian.