Good spot regarding this gap and I believe it was indeed for unbuilt 314s. I'm sure it was planned for a larger fleet of 314s (i.e. 33 units) to provide all Argyle Line workings and a number of the North Electric services from Queen Street low level, too. In the end, rationalisation of Strathclyde services and cooled ambitions allowed the bare minimum of 16 sets to be constructed with 303s and 311s drafted in to cover some Argyle Line diagrams from day one. It wasn't long before the 303s, 311s and 314s became common across all low level workings; the 314s had a lot of work via Queen Street before the 320s arrived.
The same is true of the 507s to an extent. The plan for 47 was shelved long before a measly 30 were ordered by BR in a last-minute scale back from 38, hence the 64400-64404 gap. Reportedly PTE pressure (and possibly extra funding) permitted an additional three to be built, but after the 508s. For a while it was planned to retain a number of 502s (17 I seem to recall, to cover for the unbuilt 507s) and tart them up, keeping them for Kirkby-Garston and peak Southport and Ormskirk runs until additional replacement stock was sought. Ultimately a combination of Class 503s on Kirkby-Garston, the additional 507 trio and the early arrival of the first 508s from the south put paid to this idea and the 502s bowed out in 1980.
There is also some confusion over the 508s always being intended for Merseyside. I think the class designation itself was envisaged as Wirral Line replacement stock, before a change of plan saw them built for the Southern. However, once delivered to Wimbledon as four car sets, it was uncertain for some time whether they would actually end up in Liverpool.
There were talks between BR and Merseyside PTE to negotiate before, wisely, the 508s did end up fulfilling their previously anticipated role of seeing off the 503s. The 455/7 fleet was then ordered as a direct replacement for the 508s and incorporated the redundant 508 trailers. A proposal to fit passenger door controls to the 507s to mirror the 508s was, yet again, kyboshed on cost grounds, the 508s instead having their door controls plated over on transfer. It's a shame because the lack of passenger door controls was always one of the main criticisms of Merseyrail's fleet.
Looking at the planned service intervals for the Picc-Vic project, many more than 17 units would have been required. It was seemingly never confirmed whether the 316s were to be 3 car, 4 car or a combination of both. With Picc-Vic already shelved by 1977, the numbering gap is much more likely to have been for unbuilt 314s than 316s.
Apologies for going slightly off-subject but I hope the above is of interest to some.