No! the reg is not from the 1250 Dominator, although the valve (larger part of the two stage reg) body is exactly the same the exhaust valve stem is larger in diameter and the exit hole is also larger, plus also the exhaust valve spring is shorter and has a softer spring rate, and the bevillelle washer's are stacked in a different order.
You can if you wish run the AR20 of just the larger reg by removing the smaller first stage part, this is only used to reduce the air pressure from 300bar down to 200bar (remember what these rifle's were designed for, indoor comp's 300 bar = 400 shots) You can then only fill to 200bar but this will still give you 80 - 100 shots depending on the size of you air cylinder.
Care must be taken when taking apart the reg's as all the threads are locked tight with an extremely good locking compound that can required some heat, to much heat will melt the pollyurathain seals and the sizes are a bugger to get hold of.
As to servicing a AR20, there's not a lot to do and it's a very simple rifle to work on, and if anyone tell's you that they are made of monkey metal what a load of bull! The Ally shroud and breach are made from top quality Ally, just pick up the breach on its own and feel the weight or ask the Guy who milled my shroud for me. (Phone No supplied) even the weights in the stock are all Ally.
I have read elsewhere that someone suggest that the sears on the trigger would wear out due to the parts being cheaply made, again bulls@#t. My AR20 is over three years old and has been used for HFT almost every weekend, so it's had several thousand's of pellets through it and there are no signs of any wear on the sear's nor have any of the seal's failed.
The only faulty part is the design of the reg and they tend to only last about 3yrs before the start to lose pressure. A new one can be had for about €95 from Gremany or I have heard that you can get them for £66 exchanged but the UK importer won't have any until the new year.
Steyr Hunter HFT/ HW 100 barrel + s111 & SS buttplate. HW100Kt+ SWFA ss 10x42......Shooting HFT for MVAC.