I used to have to switch the ground on the Deluxe to correct the ground to avoid getting shocked.
We used to do a ground check on all our equipment back then before a gig, something most guys today don't know about.
("26" would mean the 26th week or approximately June).
These date codes will contain two letters of the alphabet which refer to the year and month of production, and may be interpreted as follows: In the absence of the rubber-stamped date codes, EIA numbers taken from the transformers may allow you to determine the date of production of your amp.
These numbers always begin with "606" , and are followed by three or four digits in various combinations.
The first letter is the year (A=1990), and the second letter is the month (A=January). You might have to remove the chassis to be able to see the sticker, or use a mirror on a telescoping arm.
Sometimes the sticker isn't there, so the numbers on the transformer might be your best bet as Retroverbial suggested.
Fender used a conventional double-throw switch for this feature on the original blackfaces and silverfaces up until circa 1971, when a triple-throw switch replaced it.