Short answer is I don't know with certainty. ?I did go through the Keystone index to see if there was information for years earlier than 1929 without success. ?
If you're not already a member, consider joining the PRR Modeling Yahoo group. ?There are a number of early 20th Century modelers who post there on a regular basis.
While a secondary source, Tom Busack wrote about building a pair of Bowser E6s engines in the December 1990 Model Railroader. ?One of the two versions he built was an as-built 1914 E6s (ie not too far after your preferred era); he elected to follow the 1929 painting and lettering information. ?
From: "jbopp007@... [yardbirdtrains]" To: yardbirdtrains@... Sent: Friday, September 8, 2017 10:11 AM Subject: [yardbirdtrains] Re: Painting Your PRR Steam Engines
?
Doug,
Do you know how far back this standard goes? ?My period of modeling interset is the 1900-1910 period. ?I have a Gem Pennsy F3c 2-6-0 that I would like to paint. ?I know this isn't the usual subject of interest for this group but it's a nice model. ?It represents the prototype as built and I'd like to paint it accurately. ?