I have just got the details of the prices for my next batch of 'M21' 720 pistons..Note these are forged not cast, have a modern ringset and are top quality components. You will need an 'unlinered' later M20/21 barrel to go with the piston..linered types are not suitable, and an M21 crank assembly.
However, that piston spec. is going to cost £151 plus shipping. Not that bad actually when OE B33 pistons are fetching £100 on e bay and Bantam John is selling OE B31 pistons for £125..Obviously buying in a minimum order batch costs a considerable amount of money so I will need to have firm orders for 6 of these to recover some of the cost, before I can commit to a batch.
The pistons are 'dual purpose' and can also be used to enlarge the B33 engine to 570 or 600cc depending on crank spec. and are ideal for tuning applications in that type (which is what I primarily use them for) or just taking the opportunity to enlarge your standard engine if you are on +.060" and needing a rebore
Delivery is 4 weeks from the date of order placement with the supplier.
If there are any questions regarding the spec. or applications please mail me direct....Ian.
email (option): ian@wright52.plus.com
SRM generally use good quality components but I have no idea where they source them from..perhaps they have commissioned them themselves from one of the better manufacturers?...Ian
email (option): ian@wright52.plus.com
i may be interested in one of thoes after when i start my rebuild but not till after cristmas if im not pistnbroke 
regards martin
email (option): jmwyatt@btinternet.com