I was just inspecting my 84 looptail viper when I notice the number 500 stamped on the dropout.
Yes the full serial number is on the BB.
My question is.what does this 500 mean
Serial no. is F4126916
TT 18.25 in.
DT 18 in.

sounds like the frame is either actually a Pacer 500 frame (a step above a Viper), or maybe a factory mis-stamp.

Spot on,

Early Pacer 500's were stamped 500 on the inside of the rear dropouts. It was replaced by the Super Viper (exact same frame) because DB felt the name was impacting on sales to they went back to a name more associated with DiamondBack (Super Viper). You'll find some later 500's stamped either 500 or CRMO.

F4 equated to Taiwan (Formosa) 1984, the rest is the frame number.

http://www.retrobsession.com/pages/Diamond...Pacer%20500.htm

Mark