VX1 master cylinder rebuild

I noticed that my VX-1 front brake master cylinder had a small leak.
I did some reading about hydraulic brake systems and found that the recommendation is to change the brake fluid every 2 years! Yikes - haven't touched it since it was new in 2008, so...17 years.
Finding the appropriate master cylinder rebuild kit was difficult. Brembo was of no help. The cylinders are stamped "7D10" and Brembo usually encodes the piston diameter in the part number, so I thought it must be a 10mm bore. nope. It's 12mm.
Anyway, I eventually found a kit available from a retailer on eBay (AMR-1984)
The part number is listed as Ø12mm 061046 I took a chance because there are no other 12mm kits for Brembo that I could find. It was correct.
Their website is amrricambi.com
When I did a search for 061046 on their website, 3 products came up, all referred to by the same part number. The one that I think I received is 12mm x 38mm (That's the diameter and overall length of the piston.)