The REL Mk1A telescope was simply a Mk1 telescope with Mk2 1MoA clicker plates - and without the other refinements that differentiate a Mk 2 from a Mk1 tele. Nothing more or less and nothing to do with lenses, optics or mechanics that would stand up under fire. Unless you have anything to substantiate this.

At a guess, I would venture to suggest - and this is only a guess on my part, based on my fondness for the bleedin' obvious, that the Mk1A was introduced in order to use up he back-log of awaiting Mk1 telescopes and/or tubes while the Mk2 was being prepared for production. This is based on a) the fact that the Mk1 and 1A serial numbers are pretty intermixed and b) that you can very clearly see that the letter 'A' has been added due to the slightly different font and mis-alignment