Tight Jeans: Position Lights As The PRR Intended

We all know the PRR Position Light signal looks like this with the round upper head and the (usually) squared off lower head. Both backing targets are technically interchangeable with each other (Although I have never seen a lower head type target used on an upper head.). 

However what would you think if I told you that this concept of the PRR PL did not reflect the original vision of the author. A while ago I wrote up a post discussing the compact position light and how the lamp arms of a US&S pattern position light can support a PL lamp module at any point along their length.  At the time I thought that there were two standard PL setups, full size and compact. Well it turns out there is a third method of lamp arrangement which was actually the standard setup used by the PRR in its final few decades. 

While the PRR was an industrial behemoth that didn't hesitate to spend on quality, there were a few areas where its famed standardization turned more towards penny pinching. One of these was the configuration for the lower heads of its position light signals. To make the signals stand out in bright sun, a dark backing target would be necessary. However there would be little need for this contrast if an approaching train was moving at slow speed below 30mph. Therefore the PRR only applied a backing to the lower | position as the lower / and \ positions would be approached at something more like 15mph. 

If you look closely at one of these original pattern lower heads you'll also see that the modules for the / and \ positions are not positioned at the ends of the mounting arms. Instead they are scrunched in couple inches. Although not to the level seen in compact PL arrangements, this scrunch may have been to provide some contrast without the "expense" of a backing plate. 

See the difference between the placement of the lamps on the | vs the \ position.

I am sure the rivet counting modelers are yelling at their screens right now about my obliviousness, but somehow I had never noticed this before. I guess this is partly because unmodified PRR position lights have become so uncommon, but also because later lower backing plate thoroughly set the expectation that the spacing would be the same on both heads. Anyway now I know and now you know as well.