Thank you all who helped with the serial number mystery.
Since I last posted I came to the realization that the 475 had
2mv/div sensitivity instead of the 5mv/div of the 475A. Since my
primary work is with professional audio equipment where we are
looking at low level signals (power supply noise, and subtle
oscillations, for example) I've come to the conclusion that I should
be looking for a used, late model, pristine condition 475 instead of
the 475A. I have found our that on 475's that SN of B-250000 and
upwards are 1977 vintage or newer according to the front page of a
1977 Revision 475 service manual CD being offered on ebay.
I've already searched for posts on 475 updates, so I know what to
look for re: the rectier, power resistor and PSU capacitor. Are
there any warnings that you guys can offer re: my direction to pursue
a 475 instead of the 475A?
Thanks for your input!