2020 update sort of. Spring test drive, no work done since 2019, and all 4 gears would punch when I hit the gas hard, i.e. no slip. Tried 5th and it seems OK on the flat, and previous 5th gear slippage was up a long hill which I didn't try.
So back up on the lift, pulled the inspection cover, and all is still nice and clean in the bell housing like I just installed it. I tightened up the sump bolts while there for a couple of drippy looking bolt heads. Cleaned up road draft oil. Checked W58 level and it was nearly a quart low - took almost a full pull of fluid in my vacuum pump. Valvoline synthetic 75W90. So at this point I'm doubting anything is out of sorts with the hydraulic t/o or setting, thought so but had to verify.
That brings me to the last change point before I noticed slipping. Just rebuilt clutch master cyl, might have bollixed setting the push rod and had it too tight, and it was hard to bleed properly with one person and my Motive pressure bleeder doesn't quite fit the reservoir so it leaks a little pressure. Pump it up, quickly duck under and hit the bleeder before I lose pressure - funny really! Could the spring in the m/c be broken? I might just rebuild another spare to have ready regardless.
So my solution/answer is no answer, just an update. If I get someone to assist bleeding, I'll try it again. But for now, I feel safe staying close to home at watching for slippage. Maybe it was the gearbox low fluid?
Well that's the update so as it is.
Pete