Down tube shifters have not been fitted to most road bikes since the early-mid 90's when STI / Ergo shifters became common place; as such bikes are not designed for DT shifters; all that will be on the down tube will be a cable guide, either as a braze-on or boss.
Although you can get DT shifters still very few people use them, and you will normally only be able to fit these on steel frames, the Surprise you have is an Alloy road frame (the Surpise Pista being the Track version).
Would suggest getting STI or Ergo shifters as this is what used now, and was the orginal spec for your bike.