I have an older plain 645 it's similar to the J, also have a 1000S, but I doubt I've used a shutter speed faster than 1/250 in 30 years. My cameras both need servicing unfortunately the 1000S went to Johnsons Photopia twice but came back with more issues than it went with plus damage.

These are real work-horse cameras I used mine day in day out professionally for many years and I bought them second-hand. I prefer two cameras over inter-changeable backs.