Mk II, Mk IIIA, etc. Search didn't help (2009)

I have a '73 850 Roadster undergoing rebuild. I've seen several references to Mk II, Mk IIIA, etc. Can anyone tell me what these mean and how do they relate to my '73 Roadster. Any help appreciated. signed......Newbie

As it is a "73" Roadster, then it is most likely a MkI model, with peashooter silencers/mufflers and metal airbox?
The "A" (IA & IIA) models can be identified by the annular discharge silencers/mufflers, commonly known as "black cap" (in the UK) or "bean can" (in the US) and large black plastic airbox.

Very few, if any, MkIA models were sent to the USA, as the A versions were low noise emission models intended for the European market where new regulations were being introduced. They were only made for a short while at the very end of that year ('73) and were soon replaced by the very similar IIA models for '74.

The majority of US models for '74 were MkII, which still had peashooters and metal airbox.

All the 850 MkIII models ('75-on) had black caps and plastic airbox originally, so although technically they were built to 'A' specification, they were only known as "MkIII".

Over the years, many A models will have had their black caps replaced with peashooters, and many will also have had the plastic airbox removed, so it isn't always easy to identify which MkI & II models were originaly built as 'A' models.

Thanks for the response. Mine came with a metal front and rear plates on the air cleaner which 'sandwiched' the perforated metal-wrapped element. Also it was an early serial number. This clears up a lot of my confusion and will make it less worrisome when ordering parts. Thanks!