The only problem in trying to identify Ossas without seeing them is that as the bikes were so similar between 74 and 79, if previous owners have repainted or modified them in any way it is almost impossible to tell which model it is from a description.
71 - 73 bikes, the MK1, would have had fibreglass tanks with broad green stripe as Big John said, bolt on carb, flat fork bottoms and a short wheelbase due to a shorter swing arm. You can tell the short swing arm as the shock mounts are virtually right over the spindle.
Mk2 74 - 75 had the alloy tank, rubber hose fitted carb larger diameter crankshaft assembly, the toolbox built into the left hand sidepanel and a 1" and a bit longer swing arm (spindle sited rearwards of the shock mounts) Different forks were fitted with bit where the spindle passes through now shaped and rounded. Thinner green tank stripe came down from the filler cap and swept back along bottom of tank to rear - very different from MK1. Detachable rear 'cherry bomb' shape silencer added. 310 model also introduced with red / green stripes.
MK3 76ish was a MK2 with longer front forks and the shocks canted over at more of an angle, the rear shock mount is down by the bottom rear sidepanel screw. They were then fitted with a longer front pipe followed by an hideous, longer rear silencer which was now welded to the middle box.
77 - 78 model was green tank and sidepanels, black frame, white / tranlucent (and very breakable) mudguards. New style fork yolks painted black. Around this time they moved the rear top shock mounts up the frame a bit, shortened the swingarm again and slightly changed the shape of the tank and sidepanels. The barrel fins also had cut outs in them making them look different from earlier models.
79 model got a green frame and mudguards and somewhere along the way got different porting (so I'm told but that may also apply to 77 - 78 model)
80 Then came the Gripper......
Anyway, enough of this waffle, there is a good site for Ossas with loads of info run by a Swedish guy called Mats Nyberg, link below. If you have the engine and frame numbers - they are identical 6 figure numbers except engine is prefixed M, frame prefixed B - there is a link on the site to a reference sheet by a Giovanni Dughera where you can check the model type against the frame number. As engine / frame number on a bike always matched, you can date both the frame and the engine from this chart. The early bikes are listed as MARs but the later bikes are refered to as model type TR, ie; 250 TR77 would be a trials 1977 model etc.
One thing to be aware of though is that parts for Ossas are becoming harder to source in the UK although via the web you can get virtually anything from the USA.
Hope you found something useful in all this waffle, I'm not an expert on Ossas, just that the MAR is my favourite trials bike (most of the time..) As someone else said, Keith Horsman who lives accross the road from John Lamkin's place also knows a fair bit about them and used to modify them for Ossa UK.
Link to the Ossa site is Mats Nyberg Ossa
