If the pots are original they are usually the best single reference for dating a Plexi era amp as Marshall didn't seem to buy in as large bulk quantities as the other components. You might see caps dated 65 in an amp from 67 but generally the amp would likely been made within a few months of the oldest pot date code. I will say however You really need to consider everything to get the best estimate. Serial numbers are all over the place during this period and can be very misleading. I have 11185 and 11187 (edited) and they were likely built months apart. I've seen amps that were within 10 serial numbers that were definitely were built more than a year apart. The date codes on the pots are facing down. A dental mirror and a camera works great for getting it. The image will be backwards which really makes it hard to get it right. I find it's easier to take a pic then look at it in the mirror to read it. One word of advice given earlier that is really important. Don't power it up if it hasn't seen power in several years. Have the tech pull the tubes and bring it up slowly on a variac. This accomplishes two things. First it will potentially save your OT if the caps are bad. The OT is a critical component of what makes this amp sound so amazing. The other thing it serves to accomplish is it Will reform the caps which may save them. I have a bone stock jtm45 from 1965 and a jmp50 from 68 with all original caps that are as strong as the day they were built. The key is that they have to be played. Store them in the attic unplaced for a couple of years is the worst possible thing that can happen to an amp! Regardless of when it was made these amps are amazing and incredibly inspiring to play. Enjoy!