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2) Since the label of the 400 amp main breaker is not visible, the panel directory door located on the left-hand side should have a printed label as to the manufacturer of the electrical panel. There should be a sticker located on the door.
3) If the panel directory label/sticker has been removed, I assume this panel is a main breaker panel and has individual branch circuit breakers along with the main 400 amp breaker? If so, take a look at the individual branch circuit breakers as those should have either a label or some identification as to who the branch circuit breaker manufacturer is? If the majority of the branch circuit breakers are of the same manufacturer, this will give you a good idea as to who the panel manufacturer was.
If you have a separate main service disconnect breaker and the panel is a main lug sub-panel, check the branch circuit breakers for identification and/or labeling. If the meter socket and the main service disconnect are co-located on the same enclosure, look and see if you can locate if "Milbank" is labeled anywhere. The Milbank company is the most widely used meter socket/disconnect used in the residential market.
4) If you can post a picture here, that would be most helpful. You should have a "paper-clip" icon that you can attach a picture. Please note that sometimes, posting a picture here does not work as the site is experiencing intermittent problems with posting a picture. If not able to post a picture here, go to the www.tinypic.com website and post your picture there. Once the picture is posted, reply back to me with the URL link from the website so that I may be able to view it..............Thanks.............Kevin