This is an interesting one. For many years, it was signed as "Truck Route PQ 143". Only relatively recently (within the past few years) has the PQ 143 shield (which can be seen on 2008 GMSV) been replaced by an "ALT US 5" shield. The intent with this signage is to discourage trucks from using the US 5/PQ 143 border crossing and instead direct them over to the I-91/A-55 crossing.
Caswell Ave has been a Class 1 Town Highway going back to the start of the town highway classification system in the early 1970s. Which means it's technically a state highway under day-to-day maintenance of the town, much like US 5 is through Derby Line and scores of other state and US routes within incorporated villages and cities. However, many Vermont state highways are named instead of numbered..."Berlin State Highway" on the hill connecting VT 62 near the hospital down to US 302 being one such example.
Looking at VTrans GIS data, VTrans does seem to inventory Caswell Ave as an "ALT US 5". Though I have not found anything suggesting that VTrans submitted a request to AASHTO to recognize it.
All that said, and given precedent (including routes that only have a single route sign), I'm inclined to add it as "ALT US 5" between US 5 and I-91 which is what VTrans GIS data shows. Unless someone gives me a compelling reason not to include it.
