Re: where's the tell tale? mid-section?
Based on the photos you provided, I would suggest that originally the telltale was connected to a right angle fitting on the water jacket cover between cylinder 3 and 4 (check that telltale tube length...I think its the correct length to reach). In its place the previous owner installed a temperature sender instead (see the wire there now) and opted not to have a telltale at all.
IF you wish to have a telltale, have a peek around the other side down below, and see if you have a plug on the exhuast cover. You might be able to connect it there with a right angle fitting. Pic attached is my '72 Merc.... where it connects using a platic 90? fitting. Either that, or you can remove the temperature sender if you're not going to use it with a guage.
Newer motors connect the telltale up top (better venting to avoid air pockets and hot spots on the upper cylinders), but yours does not have the boss fitting to do so. You'd have to drill it.
p.s. that tube up front is just a drain for the carbs, when they leak fuel.... supposed to be open ended like you mentioned.