After much information gathering over a few years, I thought I'd start a thread covering the differences between the models of these saws, which I will update as I go along.
Recently I've been working on an 090 for a friend (and fellow arbtalk member), and despite being the same series of saw, the differences are immense, so telling when the saw was made, or as what saw originally is very difficult.
I'll start with the clutch cover, the later model has Stihl cast into it, and the metal deflector plate on the opposite side is fixed on with 2 studs, where as the earlier (mine) isn't. There are a few other clutch covers, some have an additional ejection hole for chip, some have a location for a handle (early wrap design).
The contra has a much longer HT lead and smaller plug boot than the 090, this is because it originally mounted at an angle into the cylinder, whereas the specific 070/090 looks more like a pipe. There are only 2 boots, but colour of the HT lead varies, I have seen white, grey, black, green and red.
The most obvious difference between each model is the starter assembly. The first is a 1st generation Fairbanks-morse, whilst the other is a later, 3rd, with the Stihl logo attached (2nd gen has the logo cast into the block, fourth can be found on eBay, with the orange Stihl badge)
The on/off switch also varies, the earlier version is longer and has a simple I/O marker, whilst later versions have on/off in English and German. Some also have a rubberised cover.
I'll finish this post with a very important difference, the earlier model has a bolt that can be removed to drain the crankcase, or for a piston stop, whereas the later model has had it blanked off, this was a mid 70's change. Prior to 1967, the casing came in 2 sizes, super and standard. The super case will accept both the 58 and 66mm cylinder, whilst standard will only accept 58mm. The newest version (current) has neither of these, and the casing is much different.