Thanks for the help and suggestions folks.
As Jan-Arie's link explains (according to Google translate) the weapon itself appears to be one of NAA's contributions to precision guided munitions on the long road to what we know today as the Paveway family that have been uiniversally adopted by NATO air forces. As such I don't think it will ever have had an official designation, and even the NAA one, should any records have survived the post-Boeing dumpster, will likely be something bland like XL-Super bomb.
While being sceptical at first of Bill M's identification as an F-4D (due to the Wild Weasel/EF-4C bump antenna plate above the wing root) I'm pursuing that line of enquiry now.
For one thing the photo was stored in my F-4D folder, which must have been for some reason to start with.
But I also found out that on the road to the F-4G we all know and love (and who hasn't struggled with ID'ing all those FY69 seemingly duplicate serial number series) there were indeed four F-4Ds converted. Two, namely 66-7635 and 66-7647, with the APR-38 system's big chin and fin pods, along with its three nose and one fin lo-band black strip antennae as later used on the production F-4G conversions. For sheer ugliness thankfully those conversions weren't pursued in surplus D models, as it makes the standard F-4B/C/D/N 'donkey dong' look elegant in comparison, Here's a photo to show I'm not joking.
And then there were another two without the APR-38 that had the ER-142 or later ALR-53 'bump' antenna and respective ECM suite fitted as used on the EF-4C fleet,
F-4Ds 65-0657 and 65-0660. (Serial numbers given for anybody else interested).
'0657 looked quite likely, having served with the 57th FWW at Nellis in the early '70s. The only photo I can find shows her stripped of engines and some panels while with the 417th TFS at Holloman in the mid '70s. She was lost in a crash in 1977, so little new info is likely to be forthcoming. The only semi-conclusion I can draw is that the SEA overall scheme was not widely used in that timeframe, most common application being towards the early-to-mid '80s. One other interesting supplementary (if you find this sort of thing interesting) being that after transfer to the 37th TFW at Tonopah in 1989, the 417th became the 417 Tactical Fighter Training Squadron for the F-117A, an aircraft that specialises in PGMs.
'660 is now a gate guard at Maxwell AFB, Montgomery, Alabama and repainted in standard if glossy SEA colours including light grey undersides, and without closer inspection, betrays no trace of its former life.
Without access to the log books and or record cards, it's not yet known if either would, as Jennings suggests, have been temporarily seconded to a manufacturers trials program, but it's certainly not unlikely for a hi-performance though obsolete aircraft to participate in a hi-value Air Force program.
Special shout out to ScottsGT for the funniest post.
Once again thanks to all for your participation and interest. I'll update if I (ever) find anything more definitive.
Edited by Chek, 19 March 2018 - 09:51 PM.