I thought it might be helpful to compile a few images of the aforementioned variations of this Spiratone 18mm. The images below are not my own, but simply ones I have found across the web from forums to eBay (I have asked permissions to use these).
Here is an image of the earlier Sigma-made version of the Spiratone 18mm (with Olympus OM mount). One of the most obvious differences between the new and older design is the full circular hood vs. the newer version's dual-petal design.
I also discovered this Vivitar 17mm, which does bear striking (and by striking I do mean exactly the same) resemblance to the Spiratone 18mm here on flickr (via google).
Here is the older sigma version branded Spiratone. Notice the fully circular hood as well as the Sigma logo beside the serial. The cap of this model also brandishes a Sigma logo.(with pentax mount)
Shown below is what appears to be another version of this lens branded Asanuma 17mm in Olympus OM mount.
I will continue to update this post with images as I come across new information and unique samples.
