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Somewhere around 1976 / 1977 Hiwatt revised the phase inverter circuit, resulting in more gain. There are 4-input DR103’s from that period in circulation that have this revised PI circuit.

Right around that same time the shape of the pilot light changed from round to square.

Might the two be related?

That would be an excellent way to distinguish an early model from the revised model, without ever having to open it up to check.
So far it doesn’t look like we can tell from the serial number. E.g. up to S/N xxxxx have an early PI circuit, after that: revised PI circuit. That would be a handy thing to know.

While we’re at it, I have another question. This one concerns the Partridge transformers. I see some, mainly in early amps, with the traditional yellow Partridge sticker. Others, the later ones, have a black sticker. Are they identical, or is the one with a black sticker a ‘revised’ model that maybe goes with the revised PI circuit?

Gentlemen, I would value your thoughts on this.

I don't think the minor change in the PI affected the gain.

Only the decal on the transformer changed, the specs did not as far as I know.

The indicator lamp changed as Bulgin the manufacturer of the "bowling hat" style lamp ceased production of this lamp in 1973-1974 and supplies dried up.
There were very few if any four input amps that came originally equipped with the flat indicator lamp

I think you are confusing the four input preamp with the two input version (1978). This is when the overall gain structure changed, as well as the indicator lamp.

Thank you for your reply Dr.HIWATT.