Cannondale Question..........

I found a used Cannondale bicycle in top condition, but I am having
trouble finding out which model it is.

I think I have narrowed it down to two possibilities: SR300 and SR500.

However, in every photo I can find of these two models, the bikes look
identical.

What are the differences between these two Cannondale models?

The bike I have has 27" rims and a Suntour Cyclone II mechanism.  The
rims were made by Specialized, so I don't think they are original, but
there is a black plastic disc on the hub of the back wheel, just
behind the largest gear, that says "Cannondale" so maybe the rims are
original.  I can provide more technical details if needed.

Many thanks in advance.....