Yamaha RD125LC Model types

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There were 2 main RD125LC models ( called RZ125 in Japan) The MK1 and MK2 or LC1 LC2. However, in the United Kingdom came a 3rd model, due to the UK's MK2 being uniquely fitted with the earlier engine and exhaust.
The 3rd model then became known to the UK as the 'MK3'. It was the same as the MK2 for the rest of the world (which was now on the later production run with Gold wheels) but was power restricted and the YPVS controllers were removed.

Frame and engine prefixes
12A = MK1
restricted in carb and exhaust to 12HP

1GM = MK2  12HP
(still uses the old square barrel engine, one piece exhaust, but frame was built with mounting for ypvs servo)

2HK = MK3
Still  restricted, was now fitted with the round barrel YPVS engine, new two piece exhaust, bigger carb, but no YPVS controllers were fitted. Gold wheels only and new white paint scheme available with blue and yellow stripes.

10W = MK1
full power 21HP (also rest of world)
46W = MK1
restricted in exhaust only to 17HP (possibly just to lower revs for noise law)  Easily identified as it came with the smooth clutch cover later found on the MK2. This model had the superseded float bowl and tick over screw on its 10W carb. Also thicker fairing brackets found on the MK2.

1GU, 1GW, 1GX = MK2
Early production models had Black wheels.
Later frame numbers came with Gold wheels and a new white colour scheme being produced in 2 low volume runs, with blue and yellow stripes and note this scheme had a satin black frame. You'll also find these later Gold wheel bikes had heater pipes from the cylinder head to the carb.

(1GV was the Japanese MK2 but they called it the RZ125. Notable differences were the colour schemes, a strap on the seat and oval indicators)

The MK2 was actually based on the MK1. The frame has all the same mounting points with the addition of a bracket for the YPVS servo.
The main changes were a 16 inch front wheel, beefier forks/yolks, new instruments and brackets and a slightly thicker swing arm on the wheel mounting area. Conversions were simple and common back in the day, but it's possible the frame head has a slightly different angle. 

The bulk of the body and chassis parts are the same for both models. Many of these parts will be stamped or moulded '10W', although you must pay attention to the ignition system part numbers between all three UK models and remember to check the thickness of swing arms at the back so you know which chain adjusters and spindle to use.

There were some superseded small parts bolts etc found on both models, as Yamaha continued production. These are still inter-changeable and not worth worrying about except the round barrel engined bikes later came with heater pipes for the carb, meaning you'll need to blank off holes in the later round cylinder head if using an earlier or different carb. 
For faithful restorers, the later Mk1s had a new carb float bowl with a better drain screw/pipe system and tick over screw that can now be turned with a cross head driver. These are interchangeable if you want to get the aesthetics right.  

The RD125LC was succeeded by the TZR125, which used the same front wheel, forks front mudguard and clocks/instrument panel.

Thanks to ebay and family living abroad I was able to collect all of the colour schemes, here are 5 of the 8 for Europe.
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Link to an eBay page
Thanks to ebay and family living abroad I was able to collect all of the colour schemes, here are 5 of the 8 for Europe.