[John Bartley]

In the strictest sense it's not a trimmer but a slug. Trimmer infers that it's a capacitor

It's a trimmer. A trimmer is something that provides an adjustment to a tuned circuit and it's not restricted to capacitors. In the iF cans, the coils and mica caps are tuned to what ever the I.F. frequency is, but due to manufacturing tolerances, variances in parts manufacture causing inconsistent circuit loading etc, there needs to be some way to "trim" the I.F transformer. Some are trimmed with variable capacitors (usually mica compression) or with ferrite slugs, but either way they are trimmers.